So, I finally had some time to check under the bonnet. What the AC-guy meant was that the radiator fan that is attached to the water pump via a coupling is slipping a little bit and not spinning enough to cool. Hence, the engine is over heating, especially when the AC is on and at low revs. This doesn't show up on the temp gauge though and to me, the fan is spinning, but obviously not fast enough according to the AC-guy. At low revs, this also leads to a too high pressure in the AC gas lines(which smashed my evaporator) Anyone has an idea how to trouble shoot this one??? Thanks for all replies and sorry for the first post with some confusing info.
Pete