Sounds to me you have a heat related intermitted short occurring. What do I mean by this? Some where in your ignition circuit a wire is shorting to ground as it heats up. Now here is the real problem. Trying to find and solve this problem is not easy, even for an experienced mechanic. It could be a wire that is rubbing against a hot engine surface. It could be a coil which is starting to thermally breakdown. Could be a connection which is expanding, due to heat, and coming loose. Even plugs and switches can cause it. Trying to explain how to trouble shoot this is nearly impossible as I don't even know which engine we're dealing with. Post back with the engine and tractor model and serial numbers, and we might be able to give you a few things to try, if you feel you have enough electrical experience.