I have a 1985 300SD turbo that wil start up in 20 degrees F, but when I get the engine warmed up then turn it off the starter will not enguage. I turn the ingnition key and all the lights come on, the blower in the AC system starts, the electric windows will work but I can get anything out of the starter.

I usually have to wait 15 minutes and then it will work.
Last night I had to wait an hour and 15 minutes. Seeing as how it's getting colder I'd like to be able to depend on my car.

Any ideas?