I am hoping to gain some understanding of a curious quirk exhibited by my 1994 Pontiac Sunbird. When the car is driven out from a cold start, after being underway for several minutes, the temperature gauge begins to rise, steadily and fairly rapidly, sometimes reaching or exceeding the point where the fan comes on (approximately 215-220 degrees F.). The gauge remains at this high point for a short time, and then drops abruptly to around 160 degrees, after which it gradually warms up to a normal cruising temperature (approximately 188 degrees for highway driving). On rare occasions, I have known it to spike twice-the second spike is generally not as high as the first one.
I have had this car for over nine years now, and it seems to me that I have noted this phenomenon, in varying degrees, for a good part of that time. It seems, however, to have become more pronounced in recent weeks, with the high point being, at times, very close to the red zone. This is causing me a certain amount of concern, as this car has, in the past suffered from fairly serious problems related to overheating, and I do not want that particular trouble to recur.
I had some repair work done on the radiator this past summer, I have had some issues with the cap over the thermostat not being tight enough, giving rise to a minor coolant leak (I watch the coolant levels pretty closely!).
My questions are as follows: Would this temperature spiking be caused by a malfunctioning thermostat, or a bubble in the coolant system? And if so, why does it only occur following a cold start? Do I, perhaps, have a faulty sensor? Any insights into this puzzling matter are welcome.

this is normal operation is just more noticeable in the cold. say its 30deg outside for example your whole car, the rad, all the coolant in the heater core are all 30. you start the engine and the gauge reads what temp the coolant in the engine is, but this is separated from the rad and the heater core and that until the thermostat opens. as it opens it the 210deg coolant in the engine is introduced to a whole bunch of 30deg coolant and it will cycle back and forth till all the coolant is up to 180-190. its just much faster in the summer so its hard to see

The temperature swing characteristic is normal, but not to the degree you are indicating. On my previous vehicle, the temp would push to about 200-205 if I was ambitious on a cold engine, then I would see the gauge push up 1-2 more degrees, then lock at 185 (on a 190 thermostat).

My guess is that the thermostat is taking too long to open. It may need to be replaced, or there may be an air pocket that gets trapped behind the thermostat. One is a matter of replacing the thermostat, the other is a matter of bleeding the cooling system to get the air bubble out.

Thermostats are cheap, and typically straight forward to replace. I would replace it, bleed the system, and you should hopefully be good to go.

I agree, it sounds like you have both, a bad thermostat and an air pocket somewhere.

every car i have had has done this. the only thing i would worry about is the leak. that can cause low pressure which does cause problems. if the cap you are talking about leaking is the pressure cap especially

Thanks to everyone who responded. It'* good to know that this is, basically, a normal process-but perhaps it is time for a new thermostat.

the pressure cap operation is important too, if you think it may not be working or leaks, should be replaced. system pressure is what causes the water boiling point to be over normal operating temp of the car, without it the water will boil off and the coolant left isnt what does the cooling. so the car will overheat.