Random reboots: 5V line fluctations or CPU cooling?

Random reboots: 5V line fluctations or CPU cooling?

I just built an AMD system with XP1600+ with Volcano 9 HS, MSI 745 Ultra mobo, CD burner, 80G HD, network card, old graphic card.
Generic 400W PSU (iBest as labeled, came with the case).

No overcolcking till now.

I get random reboots (3 times in my first day "today"), specially if the cd is reading something.

I noticed two things:
(1) My CPU temp is relatively high: was 47C (somewhat loaded, case temp=29C) when it rebooted first time randomly. From what I read here, it is not that bad, but lower is better, so I turned the smart fan full (by inserting a jumper). Now it spins at 5500 RPM and my CPU temp droped to 43C (still not that low, but should be safe).
But I still get the random reboot trying to install OfficeXP from a CD...

(2) My voltages are:
Core = 1.71V
3.3 V = 3.28V
+5V = 4.92V (this one fluctuates a little bit, goes down to about 4.85V, specially when reading from CD). Without much activity (no CD..etc), it stays at 4.92V
12V = 12.1V (fluctuates between 12.0V and 12.16V on a regular bases, I think due to the fans.)

Now, my newbie feeling telling me that the 5V line is ****. Is that true?

Should I buy a new PSU?

In any case, I am gonna lap my Heatsink and use arctic Silver 3 as recommended here, but for some reason I do not think the random reboots will go away.

Help please.

Thank in advance..

I was under the impression that random reboots were a PSU problem and that if you're temp was too high your computer would freeze rather than restart but don't take it from me, wait for more responses first.

That what most of the people would reccomend get a antec or sparkle 400W PSU

P.S. on the side note, this should be moved to Power Supply Forum.