99 Bonneville heating/cooling problems. HELP

I recently repaired an intake failure on my 99 3800. I got it all together & put about 100 miles on it & noticed that the engine was not heating up to running temps (195). It would run about 120-140 degrees. I figured it was a stuck thermostat & replaced it. Seem to work OK for about 40 miles, then began to run cool again (120). Keep in mind I live in North Dakota & it is -10 outside.... but idling in a heated shop should do better then 120 degrees. I thought maybe the temp guage was not working but the heater is only blowing luke warm air & I can hold my hand on the heads without getting burnt. I pulled the new stat & checked it... seems OK. There is no pressure or bubbles in the radiator. If I had a vapor lock, wouldn't it get hot or boil over? It seems like a "stuck open stat or no stat at all" problem.... but that is obviously not the problem.

im having the exacty same problem with my 97 bonneville after having the intake replaced....wish i had a soultion for ya...ive done all the same things u have and on occasion it will actually heat up to normal, maybe once or twice a week...and i live in central iowa so i get alot of chilly rides to work lol, if u find anything out let me know and ill do the same

Turns out I had 2 bad thermostats in a row (what are the chances of that). I spent a couple bucks more on a higher quallity stat & problem was solved instantly. Comes up to running temp.(195) & stays there.

wow that is odd....however im willing to give that shot considering i bought the cheapest thermostat possible, ill try the high end one and see if that helps any...thanks!