Recently, when I depress the gas pedal, there's a 50/50 chance that I will actually increase the speed of the engine

It seems like the gas pedal is getting caught on something, but I haven't been able to detect what it is exactly. A few days ago I took my car to a Benz specialist - he took a quick look at it and assumed that there was a problem with the EGR. He took it apart, and replaced a screw that was missing. However, my problem is still not solved. I have traced the throttle control from the floor of the cabin to the engine, but I'm not able to reproduce the problem unless I'm driving.

It is a 1982 240D with a 4-speed manual transmission. Normally this wouldn't be a big deal, but pulling out in front of oncoming traffic is quite dangerous when you're not sure if your gas pedal is going to work properly.

Anyone ever had problems with this?