Silver Slayer

iTrader: (13)

I can not figure this out... My 99 SS has 50K miles on it. I bought it new, and until now my coolant temp guage was doing just like everyone else's. It would show the correct temp until 210 degrees and then stay there. Now, my guage will come up to 210 and start going back down when I am driving. It will go down to about 160 degrees. I still have the stock thermostat, and it just started as winter arrived about 4 months ago. I had a friend check it with autotap, but it was warmer outside and it was showing correctly. Any ideas???