Intermittent Speedometer Failure - Power Steering Assist

Just recently I have experienced a situation where I am driving and the speedometer just goes to zero. Other electronic gauges (fuel, temp, oil, DTE-countdown) still function fine. When this happens the Variable Assist Power Steering (VAPS) is stiff even at low speeds.  Then after driving some while, the speedometer comes back on and steering is back to normal. Once when this occurred, I stopped at a light and put the car in park and then ran through each of the gear shifter settings R, OD, 2, 1. Then put it back to OD and the speedometer came back on and VAPS also was working fine again.  Other times when this occurred I repeated the same thing and the issue did not resolve. When the speedometer is not working and I put the car in gear, it seems catch harder than normal and I feel a slight rumbling when I first start to accelerate. Beyond that the car drives just fine. '93 Lincoln Continental Signature Series, 62K original miles.