Sounds like a little more slack in the clutch cable might help which can be adjusted where it goes in at the lever. If it's borderline slippage, a higher load is going to make it slip more, so accelerating in 4th or 5th it's going to slip more than in the lower gears. I like to adjust mine to where it's fully released a little before the lever is all the way out, but not so much that the lever has to hit the handgrip to shift.