starting problems

A buddy of mine at work wants to sell me this 1990 Jetta for $300, only problem is that whenever the engine is warm, it wont start. When its been sitting the engine cranks and fires right away.

Anyone know what could cause this? I was thinkin maybe starter solenoid, or maybe some cables, any help would be GREATLY appreciated

Thanks
Brandon

By won't start, do you mean the starter doesn't rotate the engine at all or that it turns, but doesn't fire?