I have this same problem intermittently with the SDL and I believe it to be a relatively common problem with the '87 diesels. The best advice I have gleaned over the years has been to not worry about it. It is the sending unit. I also understand that synthetic oil helps to eliminate this problem.
'04 ML500 - 53k, Inspiration Edition, Desert Silver
'11 Audi A4 Avant - Brilliant Black
'87 300SDL sold
'99 C280 Sport sold
'85 190E 2.3 sold