No power at idle, fine at higher RPM

[amccoy]

Okay, I know this is a shot in the dark, but I have seen some pretty amazing diagnoses on here...

Background :
85 300d has been sitting in the garage for months while I rebuilt the front end and did some other stuff. By the time I finished with all that the tank had grown a crop of gunk like you wouldn't believe. I pulled the tank and cleaned it and have been driving pretty regularly without problems for about 3 weeks. I was out of town for a week and a half so didn't drive it then.

I started it the other day and got a rough idle and it died. Started again and rough idle again, but seemed to smooth out if I stepped on the gas. Backed it out the driveway and drove around the block, but nothing much happened when I stepped on the pedal. Just limped along, then engine would rev and I would go. Since I had recently removed and cleaned out the tank, I figured that my fuel filters had clogged with some of the residual gunk in the system. I had a spare set figuring I would need to change the filters again soon, so swapped them out. I still had some hesitation from idle, but seemed drivable. After driving on the freeway for half an hour without issues, parked the car and then when I started again really bad hesitation. Step on the gas and almost nothing happens, but then when the rpms start to go up it is fine. Seems like maybe when the turbo kicks in it is okay, but at low RPM no power, no acceleration, and prone to stall out.
at idle the oil pressure gauge actually drops down to about 2!!
Idle is rough with some shaking. If I rev the engine up it smooths out.
By necessity, I drove it back home and again, once I got up to speed no issues. Seemed to have normal power at higher RPM.

I can see a bubble in the inline fuel filter, but just 1 and it pretty much stays put. I don't see bubbles flowing through the fuel lines.
It seemed to have a bit more acceleration from a stop if I manually put it into low.
Any ideas on where to start?
Why does it seem fine at higher RPM, but not at low RPM?