Don't use a PC or laptop rig, when something burns out it can affect other equipment. Mining isn't profitable anyways but if you're mining for the heck of it build an external rig and connect it to a raspberry pi, or just use asic's, your choice
I would assume he is mining crypt and not SHA256. Anyway, he allready has a mining rig. Thats like:
A: My Computer does xy, how do I fix it?
B: Install linux!
Its bad advise.
Anyway, I had my GPU problems over the years and all I can say is: it sucks and there is not much you can do.
- You can try to underclock your GPU's while you dont mine, looks like you did this though
- You can also try to disable the 2nd GPU, not sure if you tried this while you check crossfire
- You can try a different approach: maybe its not your GPU. Random crashes usually indicate a problem with your RAM. It is strange though that you dont have Problem with LoL, but usually a System is way to complex to say this runs fine, but those dont. Might be luck that LoL runs
To check if its a problem with your RAM, run memtest86+. This might take a while to show an error though, I recommend testing 24h (also see
http://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions "4.4 How do I test RAM?").
I would bet on a RAM problem tbh. Chrome and Firefox are not very GPU intensive. There might be however a problem with the RAM on those GPUs. There are tools out there to check your GPU memory, but I can not recommend any, because I have not tried any. See here for a start:
http://superuser.com/questions/456435/how-can-i-test-my-gpu-memory-ramEdit: almost forgot the all-time-classic. Try switching to a different GPU if you have an old one somewhere or maybe you can get one from a friend for a day or two. Maybe you have a nice person that would try your GPU.