After driving for a while with A/C on,then park and turn off engine, when I restart 5 minutes later and put in drive gas pedal does not seem to respond, car just stays at about 600 rpm, then after about 10 seconds of this, car accelerates and then drives normally. Any ideas?

2 quick quesses, and it may be a combination of both.
1. the throttle plate is carboned up.
2. there is an accel. linkage rod under the air filter behind the fuel distributor that runs across the engine that is spring loaded, and it may be fighting the sticking throttle plate.
Of course I am assuming that your throttle linkage is properly lubricated, all the ball/sockets freely move???!!!

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Why does the problem only show up when engine is hot?

I agree with metricman on likely a sticking throttle plate. It probably only occurs when the engine is hot due to differential expansion of the throttle plate and throttle body. If there is carbon buildup in the throttle body (likely), it is even worse.

Hatch

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1980 240d 4sp
ASE Master Auto Technician
ASE Advanced Engine Performance

The other thing I have seen is the fuel pressure that leaks off when sitting hot. This is usually caused by a faulty EHA and is not cheap. (350).

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM