[s-cars] irregular temp gauge readings

[Matthew Russell]

Have replaced thermostat twice now, boiled both thermostats and both 
appear good. (opening around 87 C).

HVAC channel 51 reads correctly (86-89 at operating temperature) - 
meaning the MFTS is functioning, correct?

Symptoms are that temp gauge will start to rise as car warms up from 
cold, then will plummet down to almost nothing, then may come back up 
in 5 minutes or so (time varies).

Is the sending unit for the temp gauge located between the back of the 
engine and the firewall, in a plastic fitting with coolant hoses on 
either end of the fitting, held in by a clip?  Is this the likely 
culprit? Can someone point me in the right direction?

TIA,
-Matt, CO
92 s4