PC hangs randomly and also whenever I exit a game.

[Rage_3K_Moiz]

Hey guys. My dad's PC has been having problems lately. Here are the specs.

Pentium IV 1.7GHz Williamette CPU
Shuttle MV43V\N motherboard
NVIDIA GeForce 6600 256MB DDR graphics card
512MB PC3200 RAM stick + 512MB PC2700 RAM stick
40GB Seagate 5400RPM HDD
250GB Seagate Barracuda 7200RPM HDD
Microsoft Laser Mouse 6000 USB
Alcatel-Lucent USB modem
Generic 400W PSU with 16A on +12V rail, 28A on +3.3V rail and 35A on +5V rail
Sound Blaster Vibra 128 sound card
Sony DRU830A DVD writer

The problem is that the PC hangs randomly for some odd reason. Whenever I'm browsing a site or something of that sort, it'll just hang. Also, my dad says that whenever he starts a game on it he can play it for hours and not have a problem. But as soon as he exits, it won't even go to the desktop and it'll just hang. I tried it out and it's happened for all the games on his PC.
The PSU was originally an Antec 500W one but that got screwed and I replaced it with a generic 350W one. This hanging problem was happening with it too so I thought that maybe it was the problem. Since he wanted a PSU ASAP (he works on this PC and a new quality PSU would've taken weeks to arrive by ship), I got yet another generic 400W PSU which is the one in it currently. I have noticed one thing though. In the Hardware Monitor in the BIOS, the +3.3V rail drops to about 2.85V or so and never reaches 3V at all. The other rails are stable.
As for the RAM, the PC originally had the PC2700 stick only, but since my dad wanted more RAM, and PC2700 wasn't available here anywhere, I had to get a PC3200 stick. The PC's worked fine since I did that (about 3 years or so) and I ran memtest86 as well and it didn't turn up any errors. I'm baffled, so any and all help is appreciated.

 

[Rage_3K_Moiz]

^Can no one help? My dad's getting pretty pissed here....

 

[Rik]

Did you run memtest on each stick individually AND with both sticks together?

 

[Rage_3K_Moiz]

Thanks for the reply. Yes rik I did. Funny thing is, bad RAM normally gives BSODs innit? Or is hanging also part of the problem?

 

[Rik]

Usually but not always. The only time i have seen a similar problem was with a pc running 1 stick of 100 and the other of 133. I found that one of the mobo ic's was overheating (possibly caused by the mismatched ram?? I don't know) i fitted a small heat-sink to the ic and an extra fan to the pc. It hasn't completely cured the hanging but it now happens far less often.

 

[Rage_3K_Moiz]

I think I fixed it. The BIOS has some option called 'Memory Timing by SPD" and it was disabled. I enabled it and all is well. Thanks for your help rik.