Sounds like worn piston oil/compression rings and/or cylinder walls allowing oil blow-past under heavy load. If that is the case, the only solution is an engine rebuild. If the engine is in good shape, it is also possible your engine has carbon buildup which is breaking loose and combusting. You could try to clean out any carbon deposits with an oil change and a good engine oil additive, and a good fuel system cleaner to clean the engine and see if that helps.