Anyone with the same problem I had after one of the recent patches I know it is quite annoying and sometimes causes the game to crash. Well now I discovered a quick and easy fix so it no longer freezes when you alt tab.
1) Open task manager (ctrl+alt+delete)
2) Click Processes
3) Select and right click cityofheroes.exe and click "Set affinity..."
4) Select 2 of however many processors are in the list. For best performance click the first 2 (usually 0 and 1)
5) Test it and see if it worked. The game should no longer freeze from alt tabbing!
If this doesn't work let me know! It worked for me and someone else in-game
The reason this helps the game run better is because CoH was designed to run best on dual core PCs and for anyone with more than 2 the game has more bugs and delays after the recent patch. I noticed that running CoH started using all 4 of my cores instead of the usual 2. So setting it to use only 2 will yield better performance.