VERY Sluggish on 2nd half of Tank

Hi All,
About 2 months ago, I bought a 1985 300TD that had been sitting in NC for sometime. Everything is fine with this car until I get to about 1/2 tank of fuel.

Then she hesitates and runs so sluggishly it's actually dangerous to be in traffice. Thinking I may have bad fuel or water/algae, etc. I manager to limp to a service station and filled her up with diesel. After idling for a while, all seems well.

Now this morning she did it again. Valves have been adjusted and when everything's kosher, she runs like a little swiss watch.

Does it sound as if I have some gunk in the tank that needs to be removed and then clean the tank?

Any ideas/suggestions greatly appreciated!