ps3 freezing and problem starting up

Hai guys, I have a defective ps3, i hope you guys can help me with it :).
It is a 40gb CECHH model with 3.55. When i first turned it on it ran for a few minute a then froze while playing movie, i turned it off but when i turned it on back again there was no signal going through hdmi and no hdd activity and also when i wanted to turn it off i have to press the on/off button longer than usual. I have to leave it off for at least 10 minutes before i can start the ps3 again. I decided to disassemble it changed thermal paste, remove dust etc and put it up again. I then played movie, it ran for a good 30 minutes. I thought it was already fixed. I turned it off and make it stand on it sides turned on back again. But this time it froze after just loaded the movie and the problems with starting up also came back. Also it has higher chance of turning on when after turned off using the switch compare using the on/off soft button.

I have tried to restore it to default but the problems still there. I am thinking the problem is with bad solder. Luckily my friend has a heat gun. But it could be also be of other problems. If you guys know anything please share it here :)

bumping...
Did reflow but didnt work
I have tried to update to kmeaw 3.55 before doing the reflow, it updated but when it reached 41% it suddenly jumped to 100% percent and reboot. It didnt reboot properly though it went black screen and no hdd activity, I have to turn it off and then wait 10-15 minutes before i can turn it on again and at first it seems to be running fine. When i tried to play heavy rain it never went pass the first loading screen. When i tried to exit, the ps3 beeped 2 times and then reboot but didnt reboot properly went black screen, had to turn it off.
After i did the reflow, it still has the same problem....
Anybody has any ideas?

edit: tried to install ofw 3.55 but the same thing happened, it updated till 41% and then jumped to 100%. On the reflow, I reflowed both the gpu and cpu.

what was your reflow process? ie heat levels/amount of time on the chips? what thermal compound and did u use liquid flux?

just put the motherboard in the oven at 425 for 10 mins, i got the ylod today and fixed it the same day, now i have been online installation watching blue rays and playing games and the only thing i noticed is that the fan doesn't spin up as much..

Make sure when you put it in the oven it is level.