this is driving me crazy please help. 1991 940SE sedan speedometer works interminttently, works a while, jumps around then stops.

Have you read the FAQ's that are accessed via the drop down menu in the upper right hand corner?

Lots of valuable information thanks to Steve Ringlee and the people who take the time to submit what they know and have learned.

In your case this might be interesting reading:
http://www.brickboard.com/FAQ/700-900/ElectricalInstruments.htm#Speedometer_Failures

Randy

Courtesy of the 700/900 FAQ. Probably corrosion on the wires at the sensor on the differential.

Speedometer Fluctuates

[Inquiry:] My wife's 1991 740 sedan speedometer needle simply drops to zero (as if someone pulled a socket from the wall) At this moment the odometer and trip odo also quit. Very occasionally, when driving, at highway speeds, the speedometer needle will jump back up to the correct speed, and the odometer starts working, but usually after a 1/2 mile or about 40 seconds, the needle falls back to zero and the odometer quits. Other times everything works fine for hundreds of miles. Often when starting the car after is has been parked, odometer and speedometer never work at all

[Response 1: Don Foster] Check the connection at the speedo sensor. It's in the differential cover, about 3" above the filler plug. Be sure it's mechanically secure, and check the contacts (both sensor and connector) for signs of corrosion or oxidation. As much as possible, examine the harness for any signs of a break or tear, or for signs the wire might have been cut. If it's not wiring, then it's either the sensor or the speedometer head (which is electronic). Possibly, the power supplied to the speedo head where it's mounted in the dash cluster is intermittent.

[Response: Pat Hannon] The speedometer in my 745 with 160K has been jumping around a little on the highway. The cruise control would try to follow the erratic speedometer, then disengage. I suspected a broken wire at the sending unit. What I found was a burnt wire at the sending unit. The wire had grounded out on Volvo's safety wire. I suspect that the safety wire slowly eat throught the insulation on the wiring, causing it to ground out. The plug was melted and an inch or so of insulation on the green stripe wire. Make sure the safety wire is not cutting into the two wires on the sending unit at the back of your rear end. [Response: Scott] Check the instrument cluster grounds under the dash and in the right footwell. In 88 there were some 700 series cars that would register up to 60 mph while sitting still and the grounding problem was the cause.