“Lugging” the engine.
It will coast at those revs ok, but it won’t really like to pull speed from those revs in a high gear.
Try changing down a gear at that speed to increase revs and see if that changes anything.
If it persists I’d be looking at spark plugs, then leads, then the coil pack - one at a time to hopefully save money and identify the actual problem.
Cheers
Bennie