From the problem description you have given, it seems to me that these are classic symptoms of a faulty or dirty mode-switch. The mode-switch tells the electronics the position of the mechanics at any given time. If this switch becomes dirty or goes faulty, the machine can exhibit some very weird faults indeed, including the type that you describe.
I'm not saying that the problem you have definately IS related to the mode-switch as it could also be any number of other things including the suggestion that Nigel gave. However, the symptom you've described does sound typical of the kind of problem you can get when a mode-switch is dirty so I would advise cleaning or replacing it.
If you're not familiar with this procedure then I would definately advise you get a professional to do it. Cleaning or replacing the mode-switch often requires the entire deck mechanics to be removed from the machine, and the position of the mode switch as well as the alignment of the mechanics is critical when refitting. If you get it wrong, you can end up with a whole host of other problems.
Brian