1994 Dodge 3500 cummins diesel PCM

Randomly, more so when the weather is warm, the electronics of the entire charging system will go out. The tach quits, the over drive goes out, the charging system lights come on and the batteries drain quickly. If I turn the truck off and let it set awhile, it will generally correct itself and run fine. I can't think of anything other than the PCM mal-functioning. I have had it checked by numerous shops but they can, t figure it out because it doen't occur during their analysis. On shop thought it was the alternator and replaced it. Another shop thought it was the belt tensioner and replaced it. The truck still does it. Do you have any suggestions or solutions?