I doesnt actually crash and exit, the interface just stops responding. Also it stops submitting shares, I use the 50btc.com pool and when the miner hangs the drop in mhash/s is seen quickly in the pools API. And GPU-Z shows the gpus continue running the kernels.
What is the CGMINER GPU status and CGMINER pool status? Are your other network applications continue operating when you switch user?
Use cgminer API to read GPU and POOL status.
This happens after switching user:
08/16/12 14:49:06 0x12e4 INF refreshADLInfo(): min GPU utilization: 0%, max GPU temperature: 75C
...
08/16/12 14:49:21 0x12dc SVR monitorThread(): GPU mining activity: 0% is lower than threshold: 20%, timeout of 80 seconds has been reached, will restart miner process after: 15 seconds...
08/16/12 14:49:21 0x12dc SVR restartMiner(): killing miner process: 1164
I've no idea what creates that log ...