It's probably a defective motor. You are describing the exact same problem I had with one of mine. But, I know the cause of my problem.

My brother in law was fishing with me. He'd never used a downrigger before. All was fine until we picked up to move to a different spot. When he secured the DR ball to the hook on the boom he didn't attach it to the loop that is integral to the ball but attached it to the DR clip (Doh!). I didn't notice it becasue I was busy getting the kicker up, engine started, etc. While we were running to the next spot we hit some chop. The DR ball broke free, swung down into the water at 30 MPH and spooled out backwards at Mach 10. Luckily I had my drag setting such that nothing visibly "broke".

This is where you are describing exactly what was now the condition of my DR. The decent was super super SLOW. We thought the magnets were disturbed somehow. That's the only thing we could think of because we took the thing completly apart on the boat and even called a Scotty Rep in Canada.

I turned it into my local Scotty Dealer and they replaced the motor and even upgraded the belt. FREE.

I think you've got something wrong with the motor and it needs replacement.