PSU giving up?

I suspect my PSU (300w) may be giving up. I've continually upgraded my system, and suspect it can't handle the latest addition to my rig (120gb HD).

Are these the symptons of an insufficient PSU?:

The CPU fan audibly changes speed. Its not much, but I can definately hear it - as though the correct voltage isn't being consistently supplied.

The POST reporter on my ASUS mobo actually says to me "System failed CPU test" WHILST the machine is running on occasion when the CPU is at 100%.

ASUS probe comes up from time to time telling me that voltage on certain things (e.g 3.3v 5v whatever has dropped out). However, I suspect his may be bullshit because it has on one occasion done the same for temperature (reading 100C inside the case) amongst other things.

Basically I want to know if its time to get a new power supply or could something more sinister be the problem?

My system is:

ASUS A7N8x

Radeon 9700 Pro

AMD 2500 OCed to 3200

2x512mb 3200 ram

120gb HD

3 fans

DVD-rom

CD-RW

I think thats it.

Actually I forgot to mention, despite this, the machine still appears perfectly stable.

No crashesor anything. Thats nforce2 for ya. However I would still like to get this sorted, and I don't mind getting a new PSU.