thermostat replacement?

hi guys, my wifes 04 jetta tdi is not heating as it should....gets to 190 after awhile but drops at short stops..{fueling,coffe stops}. it has 140000 on it and to my knowlege its' the original thermostat. my next question is: wheres it located & what brand do you recomend? thinking about getting one & installing it myself . thanks for the help in the past & thanks for thisone in advance! Dennis

I put together a 'how-to' here:
http://forums.tdiclub.com/showthread.php?t=264619
This is on an ALH engine, not a PD, but it should be very similar. I'd recommend the real VW themostat with O-ring, and also the flange.

But I don't think you have a thermostat problem. These cars really do cool down quickly this time of year. More likely, you need to do this modification:
http://forums.tdiclub.com/showthread.php?t=160835

Thermostats don't go bad on these cars as often as the coolant temperature sensors do. If you really do have a mechanical problem, it's more likely the coolant temp. sensor.

Thanks alot Frugality! you helped me out alot & I appreciate it. Dennis Johnson.

Cooling down while stopped is normal.