You might also try running a can or two of LM Diesel Purge through the engine.

Do not add it to your fuel tank - it will be too diluted. Disconnect the tank and run the LM neat through the engine - pix on my site. Work wonders.

Then consider adding a can of Chevron Techron to a half full tank every few months - as I recall the only additive approved by MB at the time.

Finally, add some Biobor Diesel Doctor to the tank to kill any algae which may be feasting in there, run the tank almost dry and refill it from a high volume pump where you are assured of fresh fuel. Algae are notorious cloggers.

Then change the pre- and main fuel filters. All of this is illustrated on my site, mostly in the Cars - Technical - W126 Maintenance section. Filter changing is very simple. Use a clear pre-filter, not the milky white one. That way you get immediate visual confirmation of the state of cleanliness of your fuel supply.

Performance Products, renowned for their sloppy order fulfillment, sells Biobor and LM - though I would suggest you try Gerorge_Murphy@compuserve.com first.

I would try all of these before spending money on a mechanic, unless you have solid reasons to be convinced that your chain is reaching the end of its life. A 200k mile life on the 5 cyl diesel chain, if the engine has had regular oil/filter changes, would not be unusual, as the chain's path has no sudden directional reversals, in contrast to that in the V8 engines, where life can be expected to be 100k or so if you play it safe. Upper chain rails in the V8s need regular replacement also.

If your mechanic advises the chain is stretched consider replacing the tensioner also - $50 on FastLane, whatever $ he can get away with at your local shop. Labor to replace just the tensioner is 30 minutes if you are slow.

One more thought. Warm idle s/b ~750rpm. If you are below that you can get the shakes. It's a simple adjustment - one slotted screw with a 14mm retainer nut (needs a deep socket) between the injection pump and the engine. If your tach is inoperative/crazy, change the tach amplifier - no tools needed. Its the small black cylinder - 2" tall x 1 " diameter - on the driver's fender well. Screws on and off. Most seem to fail eventually and unless you want to resolder the blighter, replacement is the quick (if not cheapest - $74) fix.

And you thought diesels were low maintenance....