Computer won't boot: memory or motherboard?

Hey all, I'm hoping to get some help diagnosing a hardware problem I'm having, because I don't want to buy a replacement part and then find it wasn't the one that was actually broken.

I recently went home over the break, and I brought my desktop with me. It worked fine the whole time, but when I brought it back here and set it up, it wouldn't boot. When I pushed the power button, the case lights came on and all the fans started, but nobody was home - nothing on the monitor, no lights under the mouse, on the keyboard, or on the wireless adaptor in the USB port, etc. It would stay like this as long as I let it, doing nothing, and most interestingly I couldn't even turn it back off by holding in the power button. I had to flip the switch on the power supply to get it to turn back off.

I tried opening it up, taking everything out, and reseating it, hoping it was a loose connection from the travel, but to no avail. I then started disconnecting things one at a time to see if I could isolate the problem. When I removed the memory stick in the 3rd slot, the computer booted.

At first, I thought 'problem found, one of my memory sticks died'. The problem is that the computer's still being weird, even with the supposedly problematic stick removed. First, it no longer shuts down properly, but instead goes into that same weird fugue state where I have to flip the power supply switch to get it to turn off. Second, even with the remaining 'good' stick the only one installed, it still has problems booting. It seems that every time I remove and reseat the 'good' stick, it has a roughly 50% chance that it will boot; otherwise I have to reseat the stick again.

(Also, the computer clock no longer keeps correct time, but I have a feeling that's because I reset the CMOS as part of my 'find the problem' flailings and now some setting is screwed up, not because of whatever hardware problem I have.)

Those issues make me wonder if the motherboard is instead damaged, rather than the memory - it seems weird that somehow both memory sticks could be damaged at the same time in very similar ways. However, there definitely *does* seem to be something with the memory, too, because while it only sometimes works with the 'good' stick, if the 'bad' stick is in there it definitely won't ever boot (I tried).

I hope it just is the memory, because that's much easier to replace. However I don't want to just run out and buy more only to find that it *isn't* the problem. Does anyone have any suggestions as to how I could narrow this down further, to tell for sure whether it's the memory or motherboard?

(For reference, the computer in question was put together about a year and a half ago from one of Father Xmas' parts lists, and runs with a Corsair 650W PS, Radeon HD 5850, Western Digital 750GB HD, quad-core 3GHz AMD processor, MSI 770-C45 motherboard, and 4GB of ADATA DDR3 ram. Also, *wow* memory is a whole lot cheaper than it was when I built this thing! )

---

@MuonNeutrino
Student, Gamer, Altaholic, and future Astronomer.

This is what it means to be a tank!

Before you start replacing parts , try removing your ram and cleaning the contacts with isopropyl alcohol and some cotton swabs, might also do this with your video card, i seem to recall reading somewhere that there was some kind of problem with some of the sapphire cards having an issue with some of their thermal paste leaking and coming out clear.

If cleaning the ram makes no difference, I would buy ram first to try, based almost solely on the fact that it's so cheap. If that doesn't work, the next step would be the motherboard. You might also try moving the ram to the second set of slots to see if that makes any kind of difference.

---

Personal anecdote:
I had similar circumstances in the past. My computer started to not boot reliably, but all the fans and lights came on fine. It'd boot up if I persisted in powering up and down a few time. And during troubleshooting, I also found one of my RAM sticks seemed to have gone bad. That is, leaving it in would pretty much guarantee a blue screen, with uptime depending on which slot I had it in. Eventually, the system just wouldn't boot anymore (with fans and lights still good). I had to change out the motherboard. Unfortunately, I had just bought a set of replacement RAM, and being a cheapskate, I decided to buy a same-ish replacement motherboard . In hindsight, I should have just ate the cost of the set of new slower speed RAM and upgraded to a newer motherboard with faster RAM to stretch the life of my system.

Edit:
The motherboard of a prime suspect from the beginning, but I was trying to avoid having to replace it for as long as I could because I knew it was going to be a real hassle. However, I came across some posts about burst capacitors and as it happens, I did see the tops of several of my motherboard's capacitors bulged and cracked. Once the motherboard refused to boot, there was no escaping reality.

---

Teams are the number one killer of soloists.

If anything, I can only add to the possible points of failure.
Your description of the symptoms comes fairly close to what I've encountered when my power supply started failing, down to the fact that reducing the power consumption a little (by removing the RAM module) made the difference between not booting at all and booting more often than not. When the rest of the system is not getting enough power to work under peak load, unstable behavior is fairly normal.
The way to confirm that is monitoring the voltages output by the PSU and comparing them to the ranges they were supposed to be in.

---