Playback of recorded material is "garbled"

Playback of recorded material is "garbled"

When I am playing the recorded information from my Panasonic Microcassette Transcriber, it sounds garbled. The tape is not spinning smoothly. It seems to be held up and then let go. I tried cleaning the heads with alcohol. The problem seems to be resolved for a few minutes and then starts again. I also cleaned it with compressed air and the same thing happened. It cleared up for a bit and then started again.