I've been chasing down a cooling system difficulty on my Grand Prix since I brought it out of winter storage back in April. I'm making progress, but I don't believe it's "right". I'm basically rebuilding the cooling system. So far I've flushed out the system using AC Delco Cooling System Cleaner, replaced the radiator, radiator hoses, elbows at the belt tensioner, thermostat, and the water pump. I pulled the cooling system down to 21 inches of vacuum and refilled it. The heater core is currently bypassed. I'll be replacing that, the heater hoses, and the heater hose fittings on the belt tensioner next month while I'm on vacation. The water pump was shot and the radiator had a lot of junk in it. In 2010 when my Nephew had the car he had the intake gaskets done, both upper and lower. When he got the car back he told me the temperature gauge was " going up and down " while he was driving. I told him it sounded like the thermostat was being "lazy" and told him to replace it. Shortly after that he left with the Air Force and that was that. In 2013 when I got the car I flushed out the cooling system with plain water, changed the thermostat, refilled and bled the cooling system. When I started driving it I saw what Don, my Nephew had been seeing 3 years prior. The temperature gauge would spike between the 210F and red zone, then drop to below the 1/4 mark then continue this with the fluctuations getting less until the gauge settled in around 2 or 3 notches below the 210F mark. But as soon as I stopped for a traffic light the gauge would immediately begin to rise past the 210F mark. I only drive the car about 500 miles per year so while it was on the to-do list I didn't make it a top priority since it didn't seem to be actually overheating. Until last December when it actually did overheat! So, now where I'm at is on a cold start up the gauge slowly rises from the bottom, 160F mark up to about 2 notches below the 210F mark. Then drops to 1 or 2 notches below the 1/4 mark. Then back up to about midway between the 1/4 and 210F marks. After about 20 minutes of driving the gauge will settle in at about 2 notches above the 1/4 mark and stay there. Unless I take the car out on the expressway. I drive in 3rd because the transmission has lost overdrive and TCC lock in. The transmission is next on the to-do list. So at 50 to 60MPH the tach reads about 2700 RPMs. Under those conditions the temp gauge will creep up to the 210F mark but not go past it. However, if I stop for like 5 minutes to say run into the store. It will heat soak and the gauge will go up to about 2 notches over the 210F mark. It immediately drops when the engine is started. I've chased down cooling system problems like this before but never on anything this new. When I install the heater core and hoses next month I'm planning on flushing out the cooling system once more, installing an AC Delco thermostat, pulling it down into a vacuum again, and refilling it. What's the possibility of this being a timing chain issue? The car does have 155,000 miles on it. As I said, I've chased down cooling system problems in the past and discovered that the timing chain had been worn. If this is the case here would I be getting a check engine light because of the incorrect relationship of the can and crank positions? Or could this be the intake gaskets again? When Don had them done he didn't take it to the dealer. So I'm wondering if it wasn't done right! Or, could this be false readings on the temperature gauge from a faulty coolant temperature sensor? I haven't replaced that yet but I plan on it. The electric radiator fans are working correctly as well. Sorry for the very long "story" but I wanted to give everyone all the facts as clearly as possible! Any help on this is greatly appreciated!! This car holds a great deal of sentimental value to me!! Thank you all in advance for your help!!