['87 300D] Misfire at 3k RPM under load

[Xavier]

Hey guys, I've seen that this seems to be the place to find answers to diesel Mercedes problems so I'm posting here trying to help a neighbor out.

He has a 1987 Mercedes 300D that has a strange misfire right around 3000 RPM and up. It seems to run perfectly fine until you hit the magic 3000 RPM when you'll suddenly start experiencing it. It also can be revved right up through the 3k range with no problems whatsoever when in neutral with no load. Now, he has thrown a lot of money at the car in an attempt to fix the miss such as fuel filters all around, replacing leaking injector seats, and rebuilding primer pump. He has also tried running some diesel fuel additives to clean the injectors and the fuel as the car sat for nearly a year (though the miss appeared before it was parked).

Here is the weird part... I was pretty sure it was a fuel issue until another neighbor of ours came out touting that he was sure it was completely electrical and pulled the computer connections and sprayed them down with an electrical cleaning solution. This caused a number of things that didn't work to start working and a number of things that did work to stop working (like the blower fan and tachometer). The owner and I were surprised at what an impact that made and we noticed during this that there was a small red relay looking device to the (if you are at the front of the vehicle) right of the computer connections that are in the far left back side of the engine compartment that when pressed a certain way would actually race and/or stutter the engine speed.

One time after jiggling this relay the miss was completely gone. For a short while, anyway. It eventually returned but it definitely made me think twice about my original conclusion and made me search out an answer all over the internet which I haven't been able to find. So I am hoping someone here can provide some insight or direction on where to head with this mind boggling issue.

Thanks in advance! And if you need more info on the car or what has been done just ask and I'll get you the info you need.