I had a Saab that occasionally did the same thing, and it was always either the thermostat or air in the system just after I had changed the thermostat. Is there a procedure for bleeding the air out of your coolant system after its been opened?
On the other hand, its not impossible that your new thermostat is malfuncitoning. Considering the damage that could result from your engine overheating, I'd swap in a new one, and MAKE SURE its a real German thermostat. Generic brand thermostats from the local parts store shouldn't be trusted. They are often of different (inferior) design than OEM quality thermostats.
Lastly, barring problems with the new thermostat and bleeding air from the system, replace the coolant temperature sensor, and also check the wiring that connects to the temperature sensor. If the insulation is brittle and cracking, you could have a short somewhere that causes the erratic gauge behavior.