There's lots of things in there that can cause problems(e.g. set the dram timings too narrow, lower the AGP signaling voltage for some reason, turn on fan speed controll etc.) but it doesn't sound like the sort of issue you'd have from fiddling in the BIOS unless you overclock the AGP/PCI bus or something(why would your dad do that?).


Assuming the problem is hardware:
Frankly your problem sounds like a GPU core(not the memory) overheat issue(causes all sorts of weirdness, flickering polygons, lightmaps not rendered(fullbright), all sprites suddenly dissapear, player models dissapear, things may get rendered the wrong way(e.g. addative or solid colour) etc.).

Can you look in your case and see if your GPU fan still spins? Can you go play a GPU heavy game so you can see if it is indeed steam or hardware problems?

Assuming steam problem: Try using the default steam skin, I had assorted weirdness in DOD with a custom one(that grey one that came with steam), text dissapeared or appereared as solid boxes(when having console up, it appeared as boxes, else invisible).