About a week and a half ago, the upper radiator hose in my Sunbird went out, and had to be replaced. I replaced the thermostat at the same time, and it seems that the temperature spiking is significantly less extreme. I have another question, however. I have been noticing, in recent weeks, that the engine tends to run roughly when the car is started cold. The degree of roughness is quite variable-sometimes the engine sounds distinctly "off-rhythm", at other times, the idle is rhythmic, but rough. The idle generally smoothes out within a few minutes, and the car runs well when she gets underway, but my concern is that this may be an early symptom of a situation that could get worse. I know that in these cases, the spark plugs are usually the first area of suspicion, but the plugs I presently have on the car have been there for less than 18,000 miles. Is it reasonable to view them with suspicion? Could these symptoms be caused by a vacuum leak? Are there other possibilities that I should consider? Any insights are welcome, and I hope you are all enjoying your Christmas holidays.

After thermostat & rad hose replacement, when cold for a few days, open rad cap & top up as necessary and ensure the overflow bottle is on the full cold line. Of course make sure there are no leaks anywhere around the hose clamps etc.
I would go through the whole car with a good tune up. New plugs & spark plug wires, air filter, fuel filter, throttle body cleaning and the IAC (intake air control) & port cleaning as well.
Are you using regular engine oil or full synthetic ?
When you say cold... I just noticed you are in California.
Come on up here for the new few weeks...going to be real frosty I think LOL..

I forgot to mention..
At your local Wall Mart it is a good idea to run a good fuel system cleaner a couple of times per year (added to fuel tank)

One ounce per gallon ratio and take it for a good hwy trip to blow the dust out.
You may notice a good smoother running engine again.
A vacuum leak would generally cause the car to always run rough.
I generally look around whenever I can, to look for broken or cracked hoses anyways.

What about the plug wires themselves?

Thank you to everyone who responded to my post. In answer to the questions that were asked, I am presently using conventional oil, (high-mileage Pennzoil), and I changed it last in late November. I actually ended up changing the spark plug wires at the same time I changed the radiator hose, and the reason why is rather embarrassing: When the hose went out, I was about 25 miles from home, and the local parts houses did not have the appropriate hose in stock, so my dad and I ended up improvising a temporary fix with a piece of flex house in order to get the car home. The flex hose was a bit bigger than the regular hose, and one of the spark plug wires was pressed against the exhaust manifold, and got burned. This raises the question of whether I might have done something wrong when changing the wires, but I was noticing the rough idling before this happened. Personally, I am becoming increasingly suspicious of a vacuum leak, as I have noticed that the idling, while even, tends to be rather hard even after warming up, particularly if the car is idling in Park or Reverse. (Were I to anthropomorphize, I would say her heart is pounding at these times.) This will be my first line of inquiry, but i will consider all the suggestions that have been offered. Thanks again, and Happy New Year!