I have the same problem with my 260E, 1988, 370 000 km.
I opened ( very carefully) the pump relay and the OVP ( both original from 1988) and found some broken weldings of electronic components.
I welded them, and replaced both FPR and OVP in the car.
Seems to work, but the probelm is so erratic , that I can't say if it has been soved.
Let me know what are your findings if you do the same.