This really sounds like a short circuit somewhere, perhaps a chafed wire, Fuses tend to blow fast, unlike circuit breakers that require high current over a period of time. I would (first buy a bunch of fuses) use the schematic to identify any unnecessary circuits that can be disconnected by unplugging connectors and pulling fuses, In this case the only "necessary" circuits are what you need to start the tractor (starter and glow). Once you pull the plug on all the unnecessary stuff (lights, radio, etc.) use the tractor and see if the fuse blows. If it does the problem is in those few remaining circuits and you've narrowed it down. If it's OK running just the starter and the glow system add the disconnected circuits back ONE AT A TIME until you find the offending circuit. When you isolate the problem to one or two circuits examine the associated wires carefully until you find the cause.
Intermittant problems stink but you have the time to really find the cause using a systematic approach. If you take it to a pro, he can never be sure he found or fixed the real problem. You can. So get out your schematic and see how many unnecessary things you can disconnect.