I have fully-patched windows 7 64bit on my laptop Asus B53J. I've been using it for a year and a half without much problems (well I had to send it to service once, they put in new motherboard). For some time now windows freezes randomly. It is a "hard freeze" - if I try to toggle caps lock, the light stays as it was. It seems to happen more when under heavy load, but sometimes it's just completly unexpected (like when browsing the web). There's neither BSOD nor any (suspicious) entry in the syslog. I tried resetting windows settings and running chkdsk and memtest. The latter two failed to run (probably because I have encrypted HDD). I've recently bought Kerbal Space Program and it ran fine for the first day, now it makes my laptop freeze after few minutes of running the game. I think it does not overheat. I see no dust inside and I tried to vacuum the fan. Also the air flowing outside is not that hot (I've managed to make it hotter before).
From what I've read it might be a RAM problem.
I'll try to download memtest image and test it ASAP. I used memtest86 to test it and there were no errors. Do you have any ideas what else could cause this strange behavior, something that can be easily tested?