audi b6 1.9tdi misfire

[lowey]

I have a 145k 2004 audi a4 avant. It has just developed a misfire which mainly occurs under load in 4th or 5th accelerating and at between 60 and 70 mph. If I join a busy motorway at maybe 40 mph and then very gradually increase speed, no problems. I live 5 minutes from the motorway, so the engine is not very warm when the misfire occurs. If it was a petrol engine, I would suspect a bad plug.... I have changed the fuel filter and run a bottle of wynns injector cleaner through the tank. The problem seem to be getting worse. The engine drives well off the motorway and accelerates as it should in urban driving.I have noticed that the trip computer is fluctuating about 15 or 20 miles with distance left in the tank when cruising at 70. The performance at speed is also reducing. I always service and repair my cars/bikes but this has me stumped. Can anyone help?