I have been having an odd problem with start ups for a few months now. My PC sometimes chugs a few times before it starts to fire up windows. LED lights will flash on, then off, once it took like 2 minutes to get going. Once it is up, no problems.

A few days ago I had a random restart. Now every time I start to play HL2 I get a restart. This happens with 30 seconds of game play, so I don't think it is a GPU issue.

I was thinking MoBo was going bad, but also thinking PSU?

Full system specs would be helpful. This could be a thermal problem, a bad-capacitors problem, or yes, the PSU failing, perhaps just weakening with age.

Gigabyte 965p-S3
C2D E6300
2 MB Gskill Ram
GeForce 8800 GT OC
250 GB HD
HP DVD
Toshiba DVD
XP

BFG 600 W PSU that I think is pushing 4 years old. Most of the system has been the same for over 1.5 years except the 8800 which is 3-4 months old. I was playing HL2 for hours until the other day.

I've been watching my temps and I don't think it's thermal, a least for the GPU.

Most likely dried out capacitors in PSU do to age/ temperature or both. Just replace PSU

Will a 550W run this system ok?

A quality one, yes. Corsair, Enermax, OCZ, Antec, PC P&C, FSP...

Good quality 550W will be fine. I have Corsair 550 VX myself with overclocked Athlon X2, overclocked 8800GTX, 4 x 1GB RAM, 2 x Hdd,2 x DVD-RW, 4 case fans.

I found an Antec 650W at Best Buy for 109.00 I'm going for that one.