It stays at midway mark when highway driving.

When you are driving there is air going thru your radiator allowing the car to run cooler.Check the thermostat.That could very well be your problem.

As long as it does not bang off the top of the gauge l would say that you do not have a serious problem. As the first poster recommended, have the thermostat checked. Your probably overdue for a new one anyway.

Some GM gauge clusters have faulty parts that break prematurely and that could be the cause but it is more likely the thermostat.
If other gages on the cluster act oddly speedometer, temp, and tachometer then you better have that checked out.
Note : the clusters seemed to always work when you take it to the dealer.