badger 8.3 cummins loses power

have a 94 badger wheeled excavator. drive engine cummins 6c8.3 loses power acts like its running out of fuel? replaced fuel filtrers,lift pump,overflow check valve? while under a say a long haul some times it acts like a governor or cruise control then some times it bogs right down ..then it picks back up..it only has 18000 miles on it?
any suggestions would be apprciated

badger 8.3 cummins loses power