87 GMC dies

Have a 87 GMC that dies under light load have a code 33 changed map sensor but no help. the truck has 51,000 miles on it the pcv hose was plug we took tappet pans off and oil pan and cleaned the slug out a lot of it.And the drain holes in the heads. Then power washed the out side of the engine.When it start to die if you step on the gass it will recover it may run 10 miles or maybe a mile then do it againe.If you don't steep on the gass it will die it start verry easey every time any help please thank you