Educaid

Purchased a 1989 E260 from a friend who purchased it from an estate. He got it with 85,000 miles, drove it for a year and sold it to me with 100,000 miles. Since the weather is now cold I am having problems getting it started. It appears to start and then loses its idle. After the 3rd try it will start with the accelerator partially depressed. I leave it idle for a couple of minutes at about 1500 rpm and after 2 minutes the check engine light comes on. I will then drive it about 2 miles and the check engine light will go out. After that it runs flawlessly and is a pleasure to drive. I took it to the dealer when I first got it because the fan would not go off. It cost me $500 for them to diagnose a faulty water temperature sensor. What should I check? It seems like it if it had just a little more cold idle until it warmed up that would fix it. Could it be an O2 sensor, voltage protection relay or air temperature sensor? I would think something like that should be easily diagnosed, but after my water temperature sensor problem I don't want to spend another $500 to find out. Thanks.