As far as I know, no RAM shuts itself off from too much heat. It really sounds like your RAM is the problem, especially since it sounds like you tightened the timings without doing a memtest to check for stability. And even without an overclock, RAM can go bad, especially no-name brand. Memtest will simply tell you if the RAM is able to perform without errors. I highly recommend running a test. A full test can take several hours, but errors usually start getting reported in the first hour if its bad.
Loosening the timings and turning off sound probably means you had too much going on at the same time for your memory. Either that or its a mobo issue, but you recently replaced it so that's unlikely. Toss 2x1 GB of quality RAM in there
If it was a GPU issue, you'd probably see artifacts before it became unplayable from choppiness.
I'm not 100% on this but a sound card MAY help. I don't know how good the integrated sound is on that board, nor do I know if a sound card stresses the mobo/RAM less than onboard, but I'm sure one of these guys knows.