Okay, finally got done fixing temperature problems in both cars.
In the 02 Jetta TDI with low temperature, I replaced the following:
1) Bad thermostat with a Behr 87 C oem thermostat. Also replaced thermostat housing, torqued to 11 ft lbs. No leaks. Broke the tabs of the thermostat housing, replaced with a new one. Next time I do the same job, I will remove the screws, wiggle the housing a bit to get separation from the thermostat and turn the housing counterclockwise 1/4 turn if at all possible. Not sure if that much turn can be done but if it can, you can keep the tabs intact. This moves the tabs away from the thermostat so that the housing can be pulled without breaking the tabs. Installation of the thermostat is easier by connecting the thermostat to the housing and turning 1/4 clockwise and installing as one piece. Now getting 91 to 94 max operating temperature.
2) Also replaced the coolant sensor with the MTC sensor from Autohaus due to sharp fluctuations of 2 to 3 degrees. Careful with removing the connector, I broke the tab on mine. Hopefully it stays on. Now only getting occasional fluctuations of 1 degree.
Hopefully will start to get good mileage on this car.
In the 00 Jetta TDI with temperature around 99 C, I replaced the sensor with the fluctuating sensor from the 02 Jetta TDI just to detect a defective sensor. The temperature dropped to 87 - 91 range. This indicates a defective sensor (a lying sensor as Powder Hound says) and possibly a slightly failing thermostat. Genesis, I found out that the fans do work so I'm happy about that. Was also worried that the radiator might have a slight blockage causing the 99C temps but that has been eliminated since the temps obviously dropped with a different sensor. Will replace sensor and thermostat next weekend when I get them ordered from Autohausaz. They have good prices as in below..
Behr OEM 87C thermostat: $14.71. Appears to be exactly the same as the one I replaced.
Thermostat Housing: $3.61
MTC coolant sensor: $9.91 Elth Coolant Sensor: $12.60 Bought the MTC which seems to work well for now. Will share experience in a few months.
Sensor o-ring: $0.11
Sensor U retaining clip: $0.11
Thank you Genesis and Powder Hound for the input. I am one happy TDI'er.