I noticed today that everytime I got on the gas and the tach would rise all the while the coolant gauge would fall below C. When I let off of the gas and the RPM 's drop the gauge goes back to normal. Is my thermostat getting stuck open? or is something wrong with the sending unit?

Try checking for a loose connector (spade) or frayed wire. That was the cause when I had a similar problem. Also a replacement sending unit is around $18.

Steve

Sounds like a temperature sending unit.