Intermittent idle problem

[Chrisheg]

'69 280SL, mechanical fuel injection, Version 1 electric fuel pump

I have a problem that only occurs after the car has been driven for some time, say 1/2 hour or more. If I come to a stop with the car in gear it starts to drop RPMs in a cyclic way. Every 10 seconds or so it lugs down to about 3000 RPMs for a couple of seconds. Eventually it stall. It idles OK if I take it out of gear. If I shut down for 10 minutes is runs fine again.

This car was driven very little for about 10 years. I have done the following things to try to correct it he idle problem:

Drained the fuel tank and lines and replaced the fuel filter
Replaced the fuel tank strainer
Added injector cleaner to the fuel
Adjusted the linkages per the manual
Adjusted the idle air screw and injection pump idle screw with a tach and a vacuum gauge.

I have the original tall fuel pump and it is making a sort of high pitched cyclic whine after running for a while.

Any advice would be appreciated.

[w113dude]

You might want to check out your fuel vapor system, if not enough air is in the tank might causes fuel starvation, take the gas cap off or loosen it so air can get in for a while & see if it'll do the same.
Other possibility is that your car might be running lean!