Erratic idle problems 97 t-bird

Hello, I have a 97 ford thunderbird 4.6L v8 with 133500 miles, last week when I cranked it up, I noticed that my idle was behaving erratically, it would fluctuate between 550-900rpm, well I didnt think too much about it since it was very cold outside. Well the past week its been doing the same thing. And today when I cranked it up the rpm while in park or neutral will stay at 1500 rpm. Usually at idle it will stay at around 850 to 900rpm. And while in drive it will lower to about 550 rpm. I have no idea what is causing this, and ford wants $100 just too hook it up to the computer. I know the engine is in need of a tune up. I bought the car with 124,000 miles on it, and the idle was rock solid. Until last week. I havent done anything unusal to it, and my driving habits havent changed. Acutally the only thing ive done is last week I finally used my cruise control, which im not sure if that has anything to do with it. Since it did exactly what it was suppost to do. Check engine light as not come on, and everything seems to be normal except for the idle. Any help would be greatly appreciated.