99 Bonneville heating/cooling problems. HELP

[PBR streetgang]

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.

[pontiacfreak_81]

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

[PBR streetgang]

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.

[pontiacfreak_81]

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!