I've had the same symptoms and it was interference from my car's distributor.

I was taking a trip in my piece "O" junk 76 Chey van and temporarily place my MKIIa in the small storage compartment on the motor cover, plugged into the cigarette lighter and put on my head phones. All worked fine for a while but then it started locking up on me and the "W" came on. When I rebooted I got a unmask error. I immediately thought the HD was going bad AGAIN and began to doubt MR. Rio's reliability. I then realized that the motor's distributor/coil was directly behind where the player was sitting. I then moved it to the seat next to me and all was happy in Rio land. As soon as I placed it near the cover it would freak out.

Maybe you are getting interference from something in your car as well.