We have had this happen to us, we thought it was a faulty switch also, we found out it was the DNS that was playing up. We corrected the problem by deleting and rebuilding the forward and reverse lookup zones. Things went fine for a couple of months, but it came back, in the end we had to rebuild the whole of the dns from scratch and repair part of the AD (with the help of microsoft). I would not rule out DNS and advise you check it and test it properly it can be costly getting microsoft in (fortunatley I had the sense to continue our server maintenance contract, which my boss now realises was a good move as he saw it as an expense which we hardly used, so we hadnt got to find the repair bill)