there not much else it can be if its only happening in 4th and 5th. the gear doesnt change how the engine functions. OBD1 and euro 1 cars dont have the option of running multi fuel maps per gear. so the only options are, either you downshifted too soon, youre driving through water, or everywhere you drive is a 7% uphill grade.
I know a lot about cars. I can look at a car's headlights and tell you exactly which way it's coming