Engine pickup problem

[Sharan tdi]

My 55 plate sharan has had a niggling problem for the last year and wondered if anybody could suggest what the fault is. Occasionally when I apply throttle through the 1500-1800 rev range I get a little hesitation as if the engine is being staved of fuel. Most noticable when applying sligt load say uphill in 2nd gear but i think its there in all gears just not so noticable. Last year when I brought this up at the service they could not find a fault. We changed glo plugs and waited to see if it developed any worse. Well 1 year on and still the same. On the last service they did pick up that a small cable had been trapped in the air filterr, probably on a previous service. Think the cable is something to do with air mixture. But they did not really know if it was damaged as it's inside a plastic sheath.

Thanks

David