Just spent many hours today pinpointing a faulty tantalum capacitor.
It was in the display board of the SC Deep Cycle battery charger.
The charger would run for about 5 minutes, then slowly the 5V rail to the PIC would slowly drop to 3.5V or so and everything would stop, obviously.
If they used a simple 7805 regulator, without a series input resistor, the blue smoke would have been released and made the fault finding far easier :D
In this instance, obviously (now) the cap began leaking more the longer volts were applied.
Ray