I had this with the Nav in my E46 - I eventually managed to get the dealer to swap the Mk4 out under warranty and it resolved the issue - evidently it was the fan in the Mk4 Navicomp that had failed causing the thing to overheat. Having Orient Blue paint didn't help either as it was a very dark blue and seemed to suck up the heat from the sun on the rare sunny days we have in the UK.