I have a 1990 international 4600 series truck with a 7.3 litre non turbo engine and the starter motor engages intermintantly while driving any ideas?

. Faulty ignition switch? Check for wear.

. Faulty starter solenoid? Vibration causes the relay to make contact.

It shouldn't be difficult to solve.

Not sure your vehicle, many starter solenoids get power from Ignition switch, and the ground of the solenoid usually goes through safetys and interlocks. I would say the Ignition switch may be the problem or the starter wire is touching a live wire?