Temp Fluctuation while driving

Talking about a 1992 2.2L cavy. While I'm driving, weather it be on the highway or not, my temperature fluctuates from 120 to 140 degrees F. This would be acceptable at an idle, but this is happening at 65mph. The car takes normal time to get up to temperature, then once it is there, it will take about 3 minutes to get up to 140, then it drops down to 120 in about 3 seconds, and repeats.

The thermostat was replaced about 4 months ago, and just now I have replaced the radiator, (it was leaking), the water pump (it was squeaking), and the cooling fan relay (I didn't think the fan was coming on at the right time). Note: if you unplug the wire from the thermostat housing, the cooling fan does come on and the check engine light comes on, but with the thermostat plugged in, the fan doesn't seem to come on when it is supposed to.

Is this just a bad thermostat? Is the temp sending unit bad? If so, can you help me locate the temperature sending unit?

addition to the above:

while my temperature is dropping 20 degrees farenheit shouldn't I feel quite a bit of change in the air temperature coming from the heater? It is my understanding that the coolant runs to the heater core and that takes heat from the coolant to heat the air blown out the vents... so if there is a 20 degree drop in three seconds shouldn't I feel that quite a bit? Right now there is only a difference of a couple degrees during the fluctuation.

My Sunbird did that, was interesting to watch it. You may have an air pocket, it should work itself out.
Sounds like it is starting to work itself out.

Homerr

Thanks for the suggestion! However, does this sound like a blown head gasket? that is the one thing I've dismissed without actually checking out because it was replaced ~45,000 miles ago... but come to think of it, thats long enough for it to go on these damn cars

You wont necessarily feel anything in the heater. One of my cars was going from about 240-180 and it didnt feel any different.

But yeah, sounds to me kinda like there is air in the system. So i'll second that.

white smoke from the exhaust = blown head gasket?? :(

if the above is correct, does anyone have hints/directions/tips for replacing it?

white smoke from the exhaust = blown head gasket?? :(

if the above is correct, does anyone have hints/directions/tips for replacing it?

Yup, when you're at your favorite auto parts store buying the new gasket set, pick up a Haynes or Chiltons manual.

Dont worry, the OHV engine isnt real hard to do a HG on.