Adding thermal paste isn't a fix for YLOD. It's a preventative measure to try to avoid YLOD. The power supply may be the problem but usually it's the solder on the chips has cracked from heat stress and the chips aren't making proper contact with the motherboard. The only reliable fix is a professional reball. Check out www.endlesselectronics.org. Some may suggest the "Heat Gun fix" this is temporary at best and should only be used if you plan on getting a new system and want this one running long enough to do a back up or transfer.
The way I got the ylod was that I punched the ps3 because it froze on me and when I turned it back on I got ylod.
Good job. Maybe you warped the motherboard. Try another punch on the opposite side to flex it back.