Have a 1978 bronco 351m ran fine when warm. But got cold and won't start. Has new battery. Has gas and spark. But turns over slow. Took starter off and checked it. Its old but seems to be OK as far as I can see. Can it still be the starter or what?

 

Check ALL the battery cables,ground straps. Best to remove them and CLEAN them well. If that doesnt do it, check how much the starter is drawing when cranking

 

^^^ This. Part of the problem is most of the problems sort of mimic each other till you start investigating. A bad cable will seem to indicate a bad solenoid. Worn brushes in the armature might look like a weak battery. A simple voltmeter can wade through this and keep your dollars in your wallet and minimize downtime. Cold weather means everything must be serviceable and tight and charged or run the risk of no start. Spend some homework time on "Voltage Drop Testing" it's easy and quick. You'll be a pro in no time.

http://www.aa1car.com/library/1999/cm119948.htm