A while ago I got a spare cluster off of ebay that had the same problem. It would work some of the time, race to about 50 mph with the car sitting and sometimes die all together then recover. Other speedometers and clusters work fine in my car with no problems so I know that the problem this one time was definitely confined to either the cluster circuit board or the speedometer itself (I have had other grounding issues before). The ebay speedo had electrolyte from the smaller electrolytic capacitors all over the board so in that case my best guess would be that the smaller capacitors were definitely to blame. So, the fluctuating can probably be caused by both bad grounds and/or capacitors.