Bad Intermittent Vibration

I have an automatic 2002 Cavalier sedan with just 100,000 kms (61,000 miles) When I am driving down the road everything seems great, then out of the blue I get a pretty drastic vibration throughout the car; can't pinpoint where, just seems everywhere. Some serious shaking going on. So I slow right down almost to a stop and then go on my way again, and everything is back to completely normal. This has happened twice in one day.

My wife came back from driving and said it had suddenly occurred at around 85km/h,(and disappeared just as quickly; she was unable to duplicate it) so I went out in an attempt to duplicate it. Of course everything was running smooth as silk at all speeds. I increased to almost 120 km/h, and while bringing the speed back down to about 100km/h it very suddenly started vibrating badly. As I said, once I slowed down, then sped up again, everything was completely back to normal.

I am really flustered here. I don't have a clue what causes this strange behavior. I don't know if this is even worth mentioning, but the check engine light stays on; has been on since long before we bought the car. This may well not be connected at all to the vibration issue, and I would hate to throw anyone off with their diagnosis if there is indeed no connection. But it might be worth noting?

Has anyone ever heard of this happening, or experienced the same thing? Any and all help with this issue would be very much appreciated!(I live on a remote island, so am at present unable to get to a mechanic)

Thanks,
Glen