2001 Expolrer-Check Gauge Light/Temp Gauge

After startup, the temp gauge is well below the "L". After a few minutes the gauge pops up to just below the red line under "H" and the check gauge light comes on. Any ideas what might be causing this? The car still rides fine. There are 45,000 miles on the car and I have had no major repair work or accidents. This just started happening during two road trips over the holidays (totalling 1000 miles). Possible solutions would be appreciated. Thanks.