PS3 80 GB FAT problem... pls help

I have ps3 80gb FAT bought on september 2009. recently it started giving problems, it boots up fine and game also loads fine. but when i put some graphic intensive game it works for some time and then i get 3 beep and red light keeps flashing. it could not be heat cos it happens when i turn on the system first thing in morning. i thought it was the HDD and got a new 500 GB HDD, but still same issue. i have done restore, rebuild etc etc..it is very random, i was playing tekken for an hour and it worked great. it plays movies fine too.but when i play graphic intensive game it failes, like uncharted, TLOU, killzone etc etc..i heard if its ylod it wont boot up..so please help me get some solution.

Sounds like early symptoms of the YLOD. Basically what the same happened to me one day whilst I was playing a game called The Saboteur. Console randomly shut off, 3 beeps following by flashing red light, after a few more tries of it, the console eventually kicked the bucket for good and got full on YLOD. Once I found out it was basically a well known problem that has screwed many peoples FAT PS3s over. I decided to save myself the hassle and just buy a slim line PS3 instead.