Engine Temp Problem - Thermostat? Options

[Syzygy]

I had a problem over the winter with the temp gauge in my '97 ST Auto. When the car was at idle, the temp gauge would read within normal range. As soon as I started driving constantly at any speed over 30 MPH, the temp would steadily drop almost to the bottom line. My house is next to the highway, and since the auto trans doesn't shift into overdrive when the engine's cold, it would take quite a while to get it to shift, leaving me riding at 60 with the engine at 4k. This was happening even when the temp was in the 40's and 50's. Since the weather's gotten warmer, there's been no problem at all - the temp gauge gets up to about midway and stays there. Is this a thermostat problem, or elsewhere? Thanks.