Recently a family member has been having issues with a tempature problem on a 2004 Bonneville. The Tempature gauge will sometimes creep up and remain stuck on a higher than average tempature. The serious of events is listed below:

Initally it rose and platued at the 280 tempature during a highway drive. After this incident dealership recommended a thermostat replacement for a 'lazy thermostat'.

However a few days later tempature one again crept up and remained at 240. Not a serious problem but much hotter than normal. 2nd delearship claimed to have a problem with radiator cap not holding pressure and replaced said cap. Car ran normally for approximately 3 days.

Today, car went into the red on normal city driving. Approximately 10 blocks from the house. So we are talking over 280 degrees. Car didnt steam, but we believe was severely hot. I was told the dealership reprogrammed the VPM(not even sure if this is a real part) and could not get the car to recreate the problem.

Any ideas or hints on what could be causing this problem.

My ideas:
1. Bad water pump - supposedly ok
2. Bad tempature sensor - supposedly ok
3. Bad computer - supposedly ok

Incidents have occured in the following conditions
1. No AC
2. Highway and City driving.

Only remedy is simply turning the car off and on to get the tempature to reset downward. The car will sometimes drop 40 degrees simply from turning it off and back on, however this isn't always the case. Hopefully you guys can help me with ideas. By the way this car has about 37K miles. Thanks in Advance

i would start with a coolant sensor.. sounds like its on the wack...

if the car isnt acting hot when its running at 280* you have to wonder i guess

I agree with Haro that the coolant temperature sensor is the most likely culprit. Turning the car off for that short of time should not affect the coolant temperature that drastically.

can you hear the high speed fans on?????

That'* the first thing that came to my mind.... The Fans???!!! They're supposed to come on by 212F with the A/C off.... and run full time with the A/C on. If the temp sensor was out of range, the first thing you would notice is the fans comming on at the wrong times; or in your case... NEVER!!!

Check to see if you have a code stored for a water temp sensor out of range after you check the fans. If by chance the water temp sensor is good and the fans are still not comming on.... then maybe your fan relays aren't working.

All above good suggestions. Just one more thing to look at, the coolant recovery tank. Does the level vary between hot & cold engine? It is kind of a telltale sign as to how well the coolant system is operating.

May be no cooling system problem at all, except of course, faulty sending and/or gauge as suggested above. Do you have access to an infrared temperature sensor with LED pointer? If so allow the temp to rise, stop the car, and place the LED pointer on the radiator hose or block, preferably away from the exhaust manifold. I own one of those guns and find it is the easiest way to check temps. If you find the temp to measure 200 degrees F +/- 15 degrees F, or so, then you know you have a "sensing" or "reading" problem. If so, I'd start with the temp sending unit first, as noted above.

I'm busy in the process of other things, but I'll hopefully re post with the solution, whenever it is answered. By the way recently had to have the intake manifold replaced. 37K miles and just out of warranty, its insane. Its not my car, but I'll try to keep you updated. Thanks again for ideas and suggestions, I'll recommend them.