List your HBOOT version and "S" status. If you did, ok. But, I didn't catch it.
Also, list what steps you have taken.
VERY first thing:
1) Wipe cache, dalvik, factory reset, and flash modified superwipe (I've done this and in doing so have had no flashing issues. All my issues were because of something I missed or messed up).
2) Have you tried going back to a GB ROM? If so, how? Did you do a restore or a clean wipe and flash?
3) If you did a restore, did you do a complete wipe as stated in #1?
4) After flashing an ICS ROM, were you trying to restore any data from a previous ROM, whether GB or ICS?
5) If trying to restore data from a GB ROM, it will bork your ICS like it did for me, regardless of the ICS ROM.
6) If trying to restore data from an ODEX ROM to a DEODEX ROM, or vice versa, it will bork stuff.
Just let us know what was done and that will eliminate a lot of repeated suggestions.
EDIT: I'm running Booster Shot ICS with no issues or force closes. I've easily flashed my phone to different things over 50 times. Completely wiping everything like I stated in #1 has always worked for me and prevented issues in coming from a different ROM. Sometimes if you skip a wipe, it will leave remnants of the previous ROM (from what I was told and it has seemed to prove true in my cases).