I'm guessing here, but it could be the transmission speedometer sensor.
Might be a loose or bad connection or a poor ground wire.
First, check the connections to make sure they are clean and not corroded.

You say it happens early in the morning. That's when the humidity is high and condensation might have taken place on or near the sensor connection. If the connection is poor, the condensation might aggravate the situation.

This sensor sends pulses to the speedometer and also is used to synchronize part of the transmission computer system, telling it what speed is taking place. The signal is also used as part of the cruise control circuitry. If the sensor is bad or if the connection to it is poor, then both the transmission and the speedometer will provide strange reactions.

FCPgroton has a replacement for about $75. P/N 9148788
http://www.fcpgroton.com/volvo850eng_electrical.htm#coil

good luck.