Which OS are you using? IIRC, cybergraphx v4 prefers 3.1 and above.
Is your BVision overheating perhaps? They do tend to get quite hot. I installed a heatsink on mine.
Failing that, I guess you need to check your software. Make sure your BlizPPC firmware is up to date and install CGX v4 (get the latest patches too). There are a lot of env variables and monitor tooltypes for CGX, it is possible that misconfiguring these is leading to instability also.
Also, make sure your Devs/Monitors drawer only contains the actual drivers you need. I have found that having the CGXAGA driver left over from CGX v3 led to a lot of instabilities on my system.
Other things you can try is to increase the stack size in your startup sequence. That's been known to fix some odd problems.
