Strange electrical problem

Hi,

Can someone either point out my stupidity or explain the following!

My head unit has been randomly turning on and off (losing power and being unresponsive) by itself.

Doing a little bit of investigating I pulled the multi-plug out the back of the unit to test the voltage, with the probe of the multi-meter in the black earth and the other probe in the red there is 12.5V and there is 12.5v when testing the yellow permanent feed.

So if I plug the multiplug back into the stereo and repeat the above, one probe in the black one in the red, the multimeter now shows 2.3v and again 2.3v when testing the yellow!

The stereo obviously doesn't work when this is the case, so can anyone explain whats going on with the drop in voltage or what the problem might be?

Thanks

Andrew