Voltage Problems with A7V133 Motherboard

I'm having voltage problems with my A7V133 Motherboard with a 1200Mhz Athlon
Thunderbird on it. Sometimes my system doesn't even boot because the BIOS
monitor reports the +5V Voltage being too low. However Asus Probe hasn't
reported any voltage problems. Nevertheless I have the following indicators
that something with the voltage is wrong:
1. BIOS monitor reports +5V Voltage being too low, system doesn't boot.
2. Asus Smart Doctor Utility for my Asus V7700 Geforce 2 GTS once reported
that the motherboard was unable to supply the right voltage for the vga card
3. random system crashes after a clean system install, even with no load

My System specifications:
A7V133 Motherboard
1200 Mhz Athlon Thunderbird
256 MB Quality DDR Ram
Asus V7700 Geforce 2 GTS
300W Power Supply
SB Live! Gamer
Windows XP SP1

I know the power supply with 300W is a bit low, but i ran this system stable
for over 2 years now. So I don't suspect that it's a problem with the amount
of power supplied. Where do I start to look for the problem? Is it a problem
with the motherboard? With the power supply? With the Graphics Card? Any
advice would be greatly appreciated. Thanks in advance and my excuse for my
bad english.....