Temperature Gauge rising steadily then dropping suddenly

The temperature gauge needle in my Blazer uses rests on the large 1/4 mark of the gauge. Lately it has been rising up to and slightly past the 3/4 mark towards overheating and then falling back down suddenly. I initially suspected a sticking thermostat so I replaced it, but the problem still exists. The coolant is full. Any ideas on what the problem/solution could be? Thanks