Bob,

The only ways I can think that SIGTERM will come unsolicited is if the kernel
invokes oom killer.

Does the system get sluggish before slim is killed? Do you have enough memory?
If you run (in an x-terminal)

watch free -h

does the amount of free memory decrease until the auto logout?

Is there a memory hog at the top of

top -o %MEM

I will keep thinking of other options.

Mark