The first thing I'd try is cleaning both the clamps and the guide bars. The aluminum in the clamp is softer than the steel used in the guide bars and will "coat" the guide bars after some extended use. If that doesn't work, examine your clamps. Are they still squarish down at the bottom or have the worn considerably? I purchased new clamps for my 1000 after about 15 years and they held great, no slippage at all. You can expect to pay about $75/clamp. I would also give JC at Tennismachines a call. He can really provide the lowdown on any problem you might have with that machine. Good luck.
Rabbit is probably correct--if this is the first time it has happened, I suspect only a good cleaning is in order. Using alcohol, clean the glide bars themselves and then clean the portions of the clamp that grip the glide bar and the portion that grip the string. Actually, you should clean this way fairly regularly. Good luck.