Hmmm, I wonder are you use any ramlib stack fix ? I think no and this can be a source of you'r problem. Try a BlizKick ROMFix or RemApollo replacement, also there is a MCPRamLibFix in MCP package. Also StackAttack can help.
Only thing that has changed on my system is Elbox drivers.
I going to do a restore from backup and see what happens.
I running a cyberstorm card with 128mb fast ram.