I've only ever seen this problem occur with the 60s.. maybe it was due to poor manufacturing control with the launch of the system. My 60 went out and I did the reflow repair on the GPU and CPU and it worked again for another week (exactly 7 days) then kicked again. I re-applied thermal paste and reflowed for a second time, got it working again and when I took it home it 'beep beep beeeped' me in the face!
I've repaired 2 other PS3s and one is still working and the other isn't; I know this other guy that specializes in PS3 repairs off craigslist and he told me that EVERY reflow fix he's done on 60g PS3s have come back multiple times. So bottom line is if you haven't opened you system up already, just pay the $150 to sony to get a new refurbished system with a new warrantee. Or just buy a new 80 or 160 PS3 like I did, the newer ones are more stable, run quiter and cooler anyway. Chances are you wont miss playing PS2 games or using the memory card ports, you can buy PS2s for under $50 and you can manipulate memory still through the usb.