I've got a '98 Cavalier (2.2L) with some temperature gauge issues.

When driving at highway speeds (65-75mph) the coolant temperature gauge will occasionally drop significantly for a few seconds (sometimes it will peg, sometimes it will just dip). This is accompanied by a noticeable loss of engine power. When the gauge returns to its usual operating zone, the engine power returns. The CEL isn't lit, and I'm not getting any misfires. I had it checked when it first started happening, and was told that no codes were being thrown. The cabin hot/cold air is still functioning properly, so I don't believe it's a thermostat issue. If I never drove the car above 60mph, I'd never know there was a problem - the temperature gauge is responsive, and behaves normally the rest of the time, so I don't think there's a problem with my temperature sender. My gas mileage hasn't changed, and it's not eating oil or coolant.

The last time this happened, I pulled into a rest stop, let the car sit for 2 minutes while I poked around under the hood, and when I re-started the car, it behaved normally for the rest of the drive.

Any ideas?