Engine hunts badly at 2700 revs.

I have a 51 plate TD5 and this problem has just shown itself over the last 2 weeks immediately following a 500 mile round trip down to Kendal from Scotland on the M6.
It drives absolutely fine during "normal" driving and goes up through the gears with plenty of torque up to the speed limits and beyond. Overtaking on dual carriageways/motorways in 4th is no problem whatsover.
This symptom only occurs when overtaking on smaller roads where there is a distance limit with regard to getting past another vehicle which is doing e.g. 50 mph. If I accelerate to point of kick-down into 3rd gear the revs are extremely reluctant to go beyond 2700 rpm and the engine "hunts" at approx. 2 to 3 cycles a second throughout the manouvre. Once it goes back up to 4th and the revs drop back down again it takes off like a good 'un.
This is now getting scary to the point that I will just sit behind anyone doing less than the speed limit on minor roads rather than go through the stress of this problem symptom.
I immediately suspected overboost but I can hear the wastegate operating in 1st and 2nd gear when I'm driving - up to the point where road noise overcomes the ability to hear it anymore.
I have a dodgy loom to the injectors and I cleaned out any oil from the plug with solvent, which I hadn't done for a couple of months but it made no difference. I even opened the pcm can but there was absolutely nothing inside at all oil-wise.
Could the wastegate modulator still be breaking down at higher revs?
As i said, absolutely no problem during normal driving as the gear changes always occur before the problem area on the rev counter. You could drive this disco daily and be completely unaware of any problem if you didn't know it was there.
Has anybody encountered this previously or have any advice?

Cheers,

Richard.

Ok. Now that I've had time to look at this on Saturday morning it was a very simple "fix". Took 15 minutes.
The small bore vacuum tube going to the wastegate solenoid from the metal part of the intercooler hose was split just above the metal spring collar. The larger bore power steering hose had been rubbing against it and gradually widening the split until it caused a major problem re. overboost and fuel cut-off by the pcm. There was just enough slack to cut off the split section and re-attach.
The difference is absolutely phenomenal. This must have been affecting performance in ever-increasing degrees for some time.
Kick-down to 3rd when overtaking and the problem occuring at 2700 rpm with the "hunting" of the engine stopped immediately. In fact when overtaking now the gearbox hardly ever drops to 3rd at all and the "grunt" in top gear is absolutely fantastic. I've had this disco for 2 years now and I don't remember it ever having this much acceleration in top gear.

A very happy bunny.