Misfiring above 3k revs

Driving earlier I noticed the car was misfiring under load, stopped and checked the spark plugs and one of them was very loose. Did them all up and went for a drive - car seemed absolutely fine. After about an hour of driving in it, problem came back, now it seems very precise that it only occurs above 3000 revs. Its as if the car is limited and the car vibrates a lot but still keeps accelerating at perhaps a slightly slower rate. Ive checked the plugs and they are all fine now. Any ideas?

how do the plugs look? our polo did the same thing with the loose plug, then when i checked later they wernt happy at all so i put new ones in and all sorted

They all look ok, if a bit sooty

ok, id look at servicing all the rest of the ignition system, new cap, arm and leads from AVS Car Parts - [The VW - Audi - Seat - Skoda Parts Specialist] if they look pretty old.  if you run it at night spray a mist of water over the spark components that can sometimes highlight an arcing component clearly.

have a good look everywhere for air leaks too, especially in all the folds of the inlet boots splits like to hide in there

The car was serviced about 6 months ago, checked for air leaks - nothing!

was that a normal oil + filter service, or a full on with all the leads etc done? if your car has electronic ignition maybe look at the tci unit, its the 7 pin thingy in the scuttle.

Plugs and leads were done then too