It seems that one of the pieces (possible a set screw) has come loose inside of the handle of my trolling motor. The handle spins all the way around and I have to find the different speeds. It is still usable but annoying to use. What parts do I need to order to fix this problem? What is the cause of this?

SOURCE: somethings come loose in the head of the trolling

http://www.northlandmarine.com/MinnKotaQuickRef.htm

SOURCE: my brother inlaw got a minkota 40ta all terraine

I would check the battery. If you're using an old or under charged battery you will loose power for sure. Make sure you're also using a 12v deep cycle marine battery as this will ensure proper operation and a longer battery life.

SOURCE: i cant seem to figure out how to take my prop off

I had the same problem tonight. I wound up getting it off by appling a diagional pressure to the prop. That held the shaft screw. Then my Dad turned the wing nut with a neddle nose pliars conter clockwise and it came right off.

My problem and probably yours too is once inside there is a pin holding the prop to the shaft that was broken. I don't know how to get the new pin back in since it is inside the prop with no access from the outside.

SOURCE: Trolling motor stays on same speed no matter where

I just recently had a simialr problem. No speed control and motor only ran at 100. I talked to the minn kota tech rep and sent him my control board and he said it had a bad potentiometer...however, I had to buy a control board to fix the problem ($215). Hate to say it but sounds like you may need a new control board.