help! CPU temp/random shutdowns/etc.!

Hi!

I just built a new system. Here are the components:

mobo: TUL AX480A7-F socket 754 (uses AMIBIOS) 7http://www.newegg.com/product/product.asp?item=N82E16813190002

RAM: 512mb Corsair ValueSelect PC3200 (passed memtest86+ no problems)

HDD: WD http://www.newegg.com/product/prod [...] 6822144392

Video card: Sapphire Radeon x700 128mb

Power supply: Hipro "true" 420w

CPU: AMD Sempron64 2800+

This isn't the best CPU ever, but for under $400, I don't think it could be too easily beat.

So anyway, I'm having issues!

Sometimes when I restart, my computer gets past the memory test and just shuts right off. I have the CPU temp auto shut-off at 70C, but it only does it when I restart.

Let me explain some more.

My BIOS reports CPU temperature at 49C, never changing. I've tried tools that monitor it in Windows, but they always say 69 or 79C no matter what! Which just can't be true. I have a properly-applied AMD stock cooler and the system is supposed to shut down if it gets that hot anyway. Plus my case feels cool and the air being blow out of the heatsync by my CPU fan doesn't even seem that warm.

I thought it might be hard-drive related, as I've had a few problems with this one, but SMART monitors say everything is A-OK.

Every time I boot, I get 2 short beeps. I have been unable to determine what they mean. When connect/disconnecting components almost randomly (including USB mice/keyboards) sometimes its just 1 short beep.

Now, the time it kept turning off after restart is after playing Doom 3 at Ultra High Quality, 1024x768, but only for like 3-4 minutes. Before that I was using TV-out to watch a movie. And when it checked the RAM it made an odd buzzing-ish noise, I can't figure out from where.

Also, I have many performance hiccups, especially in Oblivion and Half-Life 2. 512mb was fine on my old CPU (P4 1.8GHz) for HL2 w/o the hiccups...I ordered another gig of RAM and am hoping it will clear it up.

Is there anything I can do to troubleshoot my system? Any help would be greatly appreciated. Thanks!

what power supply do you have?
what power(CNQ, temperature monitor, RPM monitor) and performance(memory latencies, FSB, PCIe(AGPx8)/PCI clock, voltages) parameters have you set?
what OS do you have and is it a fresh installation?
what drivers have you(or not) installed?
dou you have any kind of antivirus/antispy-ad-ware or other software running in background?