First, in neutral there is no load onthe engine so the problem is occurring under load.
Second, pull a couple of your spark plugs and look at them to ensure that your are running with the correct fuel/air ratio (tan) and not lean (white) or rich (black). If you are runninglean I can't help. I have the same problem and won't know the answer until later this week. If you are running rich - plugs blackened and maybe even kind of greasy - you probably have a problem in your engine temperature sensor circuit. Could be a bad sensor(not too likely)or a corroded connector at the sensor or corroded connectors where the sensor leads attach to the wiring harness about 16 away.
Need to get at least a Haynes manual and read up on this. There are several other possibilities that could contribute to the problem.