The reason you have to give it a wiggle sometimes is the rubber bushes are more than likely perished and not able to hold the motor against the driveplate. New rubber bushes will be able to support the weight of the motor and hold it in drive, which is what happens when you reach in and wiggle it back into drive. If you are not able to obtain new rubber and metal inserts you will be able to covert tube valves to suit as per my earlier posts.
Keep Well
Davy
David Selfridge