Gm, Pull the starter out, take the drive end off and blow out all the dirt and chaff behind the drive fork. Put a little white grease on the shaft. I know it sounds odd, but I'd bet my last dollar that's your problem. JOGE
Test voltage at them circuit breakers again when having the problem. When testing voltage try be on the eyelits that are fastened to studs. Another thing to try when it is bothering is have someone in cab with cab fan on high and lights on ect. Go back an wiggle heavy wires coming off of starter and wires fastened to circuit breakers. Operator in cab should quickly let you know if systems wake up. If no luck wiggle the harness from starter to breakers and breakers all the way up to cab. Some times the harness builder will not solder the eyelets to wire well enough and maybe even they spliced wire somewhere withing the harness if they came to end of wire spool while building your harness. Good luck