The Classic I/II Analog boards are notorious for bad capacitors. Even though they are the last model of the B&W compact macs, the capacitors are of poor quality. They were cheaper than the SE series for a reason I suppose.

Anyway you should be able to test this quite easily. Monitor the +5 and +12v lines idle and then during HDD activity. They will probably already be at the low side and when the hard drive is reading/writing it will get lower because the capacitors are bad.