1999 Ford F-150 pickup v-6 engine problem

engine runs fine when cold, but once it warms up, it randomly misses, and at any rpm. it will be running along just fine, then start to misfire, then back to running fine. New plugs, wires, air temp unit have done nothing. On cold days, it seems to have more power (normal) but it also does not misfire anywhere near as much as on warm days. Only codes I have seen are random misfire, and that does not show very often. Everything points to an electrical problem, but I cannot find it at all, nor can I get it to stay around long enough to find one if it is there at all. The only other problem that I can find is that it will upshift too late under full throttle conditions, having a tendency to over-rev the engine (first to second, and second to third) Of course, fuel milage has suffered somewhat. Any ideas?