engine sluggish past 45mph

What is your question?

actually an 89 v6 4wd deluxe straight shift,truck starts and pulls out okay but as you go into 4th gear about 50 mph you can feel engine lugging and it won't gain speed. sometimes if you let off the gas and then press it again it will pull out okay?? problem seems worse as it gets hot. only mode of transportation, please help!!!