Sticky Fuel Distributor in 1987 300E

Our 1987 300E has been in the shop for almost two months now with a fuel distribution problem. At first, we thought the fuel distributor was bad, but the mechanic took it apart and found the piston stuck with some sticky substance. Carburetor cleaner woundn't remove whatever the residue was and he could only get it off with degreaser. After getting the piston cleaned, he put the distributor back together and the engine ran fine at first, but then the fuel distributor piston siezed-up again. When he took the distributor apart, more of the sticky substance was there. He has repeated this process about five times total and the substance keep returning. He has also removed the gas tank and cleaned it out completely, along with the gas lines, but the substance is still there.

Prior to this problem, one or both of the fuel pumps were squealing and we were told that the bearings were going bad. After we had both fuel pumps replaced, we ran into this fuel distributor problem. I don't know if the two problems are related, but my suspicion has alway been that some kind of degradation product from the fuel pumps got into the fuel system.

Anyway, has anyone ever ran into this type of problem with the fuel distributor? If so, any recommendations on how to purge the system of this residue?

Thanks.