Try checking to see if you have AGP fast writes enabled in your BIOS
and in your video options. If it is, disable it in
both.
Fast Writes are responsible for a great number of system stability issues. Turning them off has a negligible effect on performance, yet greatly improves system stability, particularly in games.
EDIT:
Oh! And one other piece of advice I thought I'd pass along. I had a bunch of problems when re-installing Mercs a couple of weeks back. I still don't know what the real source of the problem was, but the game wouldn't load and just kept crashing.
On the re-install I ran the game once before instaling PR1 or anything else, went through the auto-config, and created a user in the game. After that after every thing I installed that changed the game (maps are fine) i.e. PR1, MekPaks, NetMech IV, I would run the game after installing each and go through the auto-config. Worked for me that way