freezing, where could the problem be? (have a hypothesis)

So my macbook 4,1 has been freezing now and then for a long time, I've been trying to diagnose the problem for a while (no warranty anymore) and found the following things:

- when it hangs short horizontal bars flash across the screen projected over whatever was showing last

- last 0.1 seconds of sounds or something like that keep repeating or there is some kind of soft ticking/thumping sound (even when no sound was playing)

- There's nothing in the logs before the reboot

- There's no bug in the ram, did a lot of random writes/reads and always came up faultless

- It's not HD related, happens on two different harddisks (one ssd)

- Also happens when running on AC without the battery in

- It's not software related, happens on different OS X installs with only basic very standard software installs, happened also at previous OS X version (use snow leopard)

- It's not external hardware related (happens also with nothing plugged-in)

- It doesn't happen only when using the WiFi (tried cable internet for a while) but maybe it doesn't happen as often (but because of the random factor it's hard to tell)

- Seems to happens more often when using certain software (vlc with certain videos, maybe dependent on the codec?) but not exclusively

- Seems to happen more on hot days but not exclusively. That seems to suggest an overheating problem but:

- Doesn't happen more often when there's high cpu usage

- Also happens when fan speed is at 1800 (lowest) so macbook self doesn't think it's overheating

- Tried using fancontrol to keep the fan speed always at a higher level (double the speed and even just as a test for a short while at the highest), this doesn't prevent the hangs.

- Happens also when using external/bluetooth speakers

So I was thinking maybe the video chip overheating? Could this be a possibility? Or do you have other suggestions? But:

- why didn't keeping the fan speed artificially high help?

- why doesn't it happen during games? granted not very high-end, but starcraft, red alert 2, duke3d, rtcw, sims3)

Is there something more I could do to diagnose?

If it's the video chip overheating how could I remedy it? I was thinking at least just open it up entirely, clean everything, change the cpu cooling paste, but do you think it would be effective especially since cpu doesn't seem the problem? (opening it up entirely seems a bit difficult and risky for a n00b like me, so not something i would do lightly - only replaced harddrive and memory so far)