I would not suggest a 'pre-emptive' replacement of the sender, since that is what I did, and it made no difference.

My symptoms are very similar to yours, and I now think it is on the gauge end, though I have not yet torn the dash apart to figure it out...

A number of other folks have also reported this problem where it has not been on the sender end, but no clear solution has been presented. I suspect they are all just 'living with it', like myself...

Sender may be a waste of $80, or so...
Better to put an ohmeter on it first to be sure.

__________________
1986 300E 5-Speed 240k mi.