91 Excel Speedometer problem-please help!

I just bought a used 91 Hyundai excel. Yesterday when I was driving to school, the speedometer needle started vibrating and it was making a loud humming noise when I went into 5th. I was slowing down and pulling over when the needle whipped around to 120mph (I was probably going about 30mph). I stopped checked things out, and everything seemed ok. The noise stopped when I did and the speedometer was back at 0. I was in the middle of nowhere so I decided to drive to a garage, but everything seemed ok - no noise, speedometer was normal. It was late in the day so I went ahead to school. The drive home was fine, too. But today, the humming (it's more of a shaking sound) is back and the needle is vibrating again but working. I obviously know nothing about cars. Is this a transmission problem? speedometer problem? Is it going to $$$$ to fix or $$? I don't have a mechanic I trust. Any advise? Please help!

my old jeep used to do that when the little spinny thing in the speedo got ice in it (cold weather). I'm a bit surprised that a 91 anything would use a spinny thing instead of an electronically controlled gauge, but hey...