97 Bonneville - around 90k miles

Yesterday I noticed it was running just a tad over normal. This afternoon while driving back to work I glanced over and it was around 230-240 at low speeds & red lights. Then it would come back to around 220 while driving 40-45.

When I got back to work I parked it and let it idle to see what would happen. The temp immediately proceeded to go back to, and stay, around normal. Maybe slightly above.

This makes no sense, if there is a problem and its running hot, shouldn't the temp go UP at idle?

Could it just be a bad thermostat?

A bad thermostat generally sticks open and the car never really gets up to temp. When the temp jumps around like that I suspect air in the cooling system. Take off the radiator cap when it'* cold and check. If it'* low, fill it up at the radiator cap.

Low speeds and red lights the car is in gear and doing work. At idle it'* not.

The low speed cooling fans come on at 216F and the high speeds at 228F.

I'd check the low speed fan to ensure it didn't burn out.

Well, here is the answer. The fluctuation was due to low coolant...which happened because the radiator is shot. So I got that replaced... Funny how the bonny always breaks in the spring around tax time. Its like it knows!

4 days after the radiator was replaced, the lifters starting ticking on start up, then went away. After about 45 mins of driving, the car lost power, started chugging, and eventually died and would not start back up. Mechanic said intake gasket blew and the cylinders filled with coolant. Is that common?? Should I be suspicious of a mistake during the radiator replacement?

Motor has approx about 90k miles

The non supercharged engine has a plastic upper intake manifold. These tend to crack around the EGR stove pipe. There are coolant ports near that stove pipe. coolant then leaks in to the intake right behind the throttle body. So ya, it seems to happen from time to time.

Now there is a ticking/diesel type sound at idle?

Can you get a video?

Have you checked your oil for signs of coolant?