1999 Mazda B3000 lacks power

The trucks has been sitting in a garage for about a year. It runs well in first 3 gears. Severly lacks power in 4th and 5th. I have regapped the plugs, put octane boost and Lucus fuel injector cleaner in and filled it with mid-grade gas in. I have also replaced the fuel filter. Still having the same lack of power issues. I don't have any signs of a blown head gasket. What else could it be?
Sorry to be so late for any response and not sure at all. It may benefit from professional injector cleaning called "Motor Vac" as a guess. IMO Lucas is a waste. If fuel problems SeaFoam is much stronger but use anything in moderation. Don't put much faith in the magic in a bottle stuff for much of anything,