Power seat with a mind of its own
 

WEIRD! While driving, my power seats in '84 300SD sometimes move all by themselves -- usually all the way forward!

I could put us with that, but now my starter won't engage sometimes --- UNTIL I play with the seat control switch awhile and find just the right spot / position of my seat... and then, it starts right up! I think the two might be related.

Any ideas?

That is doubtful. You may have a starter that is on the blink and a seat controller (if it has seat memory), if not, it may just be the switch.