Problem half-solved. I do think heat is partly an issue because after installing Speedfan, I notice the notebook hitting the high 60s, after which the only solution seems to be shutting down and letting the system cool off (or buying a cooler). I'm wondering if using the PC in a different country has anything to do with it (I'm using a voltage adapter plug and recently noticed my electric toothbrush charger has warped from the heat). OTOH, I tried running MEMTEST86 once, and after about 40 minutes the thing shutdown, although there didn't seem to be any errors. Booting up the system immediately after showed a temp of ~50, so it couldn't have been the heat. Something I forgot to mention about the memory, they are not exactly the same! I believe the original 128 is CAS 2, but the Crucial brand only offered 3.