Had the same with my Disco, the alternator belt, whilst appearing to be tight was quite worn and was able to slip because It had become narrower than it shouldve been. so whilst appearing okish- it wasnt, and intermittently lost PAS, Revcounter, and occasionaly got the charge light on for seemingly no reason.
although mine was tight it was possible with two pairs of hands to actually slip it around the pulleys as it had narrowed so much.
Might be worth looking into?
Chris