1992 E350 turbo speedo issue

[farmnfly]

I just finnished the engine swap on my new kid hauler / grocery getter. Got it on ebay, E350 with a hypermax turbo, it had a high mileage engine with oil in the coolant. I put in a low milage 7.3 I've had laying around for a while. Anyhow the guy I bought it from said the speedometer indicator would bounce (jump), he changed the instraument cluster with a used one and it still did it.
It works fine until I get over 45 mph then it will start to jump. At 60 it will bounce back to about 55 for a split second the back to 60 for a second or two then back down again. I would worry about it except that the cruse control will chase the speedometer making for a nauses ride.
I am going to swap out the pickup in the rear end first chance I get.
The only other issue I've found is the "water in fuel" light stays on even with the wire off the filter.

[PLC7.3]

Got it on ebay, E350 with a hypermax turbo, it had a high mileage engine with oil in the coolant.

Probably just the Orings in the oil cooler were cracked and leaking....

Pickup change may solve it .....also try wiggling all the connectors on the back of the cluster. Do one thing first then if no good results do the other.... Get the new pickup with the plastic covering......the old one's had metal showing and filings would stick to it and screw it up. Check wires for breaks too.

Water in fuel light problem could be a worn shorting out wire........