Engine surging
 

With my 300SD, I noticed that when I start the engine from sitting overnight it will sometimes surge a bit. What I mean by this is that the RPMs will start out around 600-700rpm for a second or two, then drop down to about 500 for a split second, then back up to 600-700. This usually happens one or two times, then it evens out. You can feel the shakiness in the car when this happens. It has the feeling that a manual transmission vehicle has when it's about to stall.

The engine runs normally and usually has full oil pressure within a second of the engine starting.

The surging doesn't happen every time either. It's completely random.

Any ideas?

I hope you don't mean 6,000 to 7,000 RPM. That is past the mechanical capabilities of the engine. I am guessing you mean 600 to 700 RPM.

what year is your benz?

Whoops. Accidentally added a zero.

1984 300SD

Being all mechanical, i'd say maybe some elements of the governor got dirty or worn. The only thing that comes to my mind that could help without tearing the IP apart , if that's the governor in fact, would be good quality biodiesel which could clean and lubricate it better
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Maybe air in the fuel lines?

The Emission Sticker on My Car has the normal idle at 750-800 rpms. So 700 rpms could be too low on a old Engine.
You could back off the Rack Dampern/Governor Idle Pin and adjust the Idle by way of the normal Idle Adjustment Nut and Screw to the Spec on the Emission Sticker. After that adjust the Rack Dampern/Governor Idle Pin to remove shaking but you don't want it to raise the Idle Speed so that your idle is being adjustec by the Rack Damper Screw.

As previousl menitoned a small Air Leak. Cheap to replace the 2 sections of Fuel Inlet Hose with 5/16 ID Hose from the Auto Parts Store; it is rated for Diesel Fuel.

The Valve in the Fuel Supply/Lift Pump could be worn or sticky and allowing Fuel to drain back to the Fuel Tank. Sort of a Pump losing prime.
If you pump on the hand primer before you start and it always starts like that that gives an ides that it is a Fuel Supply issue were the Fuel is going back to the tank.

The Fuel Pressure Relief/Overflow Valve Spring could be weak. If you have the type that can be taken apart you can do that and stretch the Spring to a 27mm free length; re-assemble and see if that helps.