1993 400E. A/C will not come on for maybe 30 to 45 minutes of driving and the engine and engine compartment gets really hot. Sometimes it comes on by itself and other times I have to restart the engine. When it does come on, it blows normal and the air is cold. Almost sounds like the heat is causing expansion of something until contact is made and it starts working again. When engine cools down, no a/c until things heat up again. I'm lost on this one. Maybe it's a wiring issue, but all mechanics want to do is to rebuild the HVAC system which is costly. Any thoughts would be helpful.