240D 82 starter motor - works sometimes!

[steeleygreg]

Have a 82 240D which the starter motor will not engage unless the purple wire is jumpered to the battery connector on the starter. It starts by using the key sometimes and others it doesn't. When the starter motor fails I've observed and measured 9.5 VDC across the purple ignition wire. When it does work, the same purple wire connection on the solenoid has 10.5 volts. It appears there is a voltage drop.
Jumping this solenoid connection with the starter/battery connector works every time, but using the key to start the car does not.
Yes it could be the ignition switch, the headlight switch, or the neutral safety switch which is causing the voltage drop. Other posts which I've read state that there should be close to 12 VDC at the purple wire solenoid connection or close to battery voltage. Is my thinking correct or am I missing something. Any help would be appreciated.

Thanks
Greg