Transmission Heat Gauge

What is your question?

I have a 1996 chevy lumina that i have had for a couple months. Every once in a while the heat gauge will go way up the high end of normal, then it will go back down to mid range. The other night going down the freeway, my heat light came on because the heat had gone into the red. I looked down at the shifter and saw that it was in drive, so i put it into OD. The heat dropped INSTANTLY. My question is this: is there a gauge that senses how hot the transmission gets, and if its bad, and could that be why my gauge is always fluxuaiting?