1992 Nissan Truck Intermittent Starts

Sometimes it makes no noise, sometimes it makes a starter buzzing (joy buzzer) noise. The strangest part is that it seems as though it is ambient temperature related. There have been times that it will start just fine first thing in the cool of the morning, and then again after I have driven all the way in to work. But at the end of the work day, after sitting in the heat of the sun, nothing. I can push it off & pop the clutch & it starts ok. So far, I have replaced the starter, battery, starter relay, all with no luck.