I would be suspect of the ignition switch... My 164 (same year, same switch) is sort of similar.. Between the off mode, and the start mode on the switch, there are about 3 different positions that will activate the starter... I just pulled the appropriate wire, and touch it to the correct one only when I need to start it...

Its an easy part to replace though...just take the plastic shroud off the column, and the switch comes off with 2 screws. Finding a new one might not be so easy, but there should still be plenty of used ones about...

Then again though... it could be a sticky solenoid too... I suppose you could ID that at the switch by crossing wires and seeing if the starter stays on when the right wire is disconnected.

Gotta fix it though... you'll waste that starter faster than you can blink an eye...and it can't be good for the flywheel teeth either!

-Matt