4x2 struggling under load

Bought it 8-9 years ago from deere, new, now only 350 hs. Never a problem. Now idles smooth but under load is struggling bad. Changed plug, fuel filter, checked pump-working, checked throttle cabble, not sticking, cleaned air filter but only minimal dirt. New plug fouled within 10 minutes--black carbon--not fuel.

Any ideas would be greatly appreciated.