Quite common. Same issue came up with my 92 400E. Fan resister was one problem (fans were not turning on when the the AC was on) Radiator Cap was losing psi so I put on a new one.... that was is! Replaced Thermastat for good measure. Flushed system and everything is OK now.

Fan clutch hasn't been an issue for me but I have heard of that causing heating probs since they seem to wear out due to extreme temps under the hood.

Good luck

__________________
Jeff
'87 300E
'92 400E
'06 ML350