TD5 Discovery Intermittent power loss

Hi having lived with this vehicle for the last 5.5 years it is now pulling harder than it's ever done before ! In fact it's a real pleasure to drive now.
When it's pulling right you can accelerate away from rest confidently, it will rev quickly and freely up to 3500 rpm and pull hard in all the gears.
The exhaust note is also different and its like driving a new vehicle.
Pull up and park for a few minutes, come back to it and the powers gone again? within approx. 1 to 2 miles of driving and the power comes back ?
There are no management lights on and there are no fault codes.
It's been de catted and does not have EGR. It's as if it can't sense load when the power has gone, is down on boost pressure or is under fueling.
Any suggestions would be greatly appreciated. It's done 140,000 miles
It had a new head gasket 60,000 miles ago, post the heat exchanger leaking coolant in to the inlet manifold. This was removed when the head gasket was replaced and converted back to the none heat exchanger type set up.

For the beginning
1. what about the injector harness/ oil at the ECU red plug? were these checked too? ... if not that's a good start

2. unplug the MAF and see how it goes then

3 unplug the AAP(the sensor in the top of the airbox) ... see how it goes

Hi sierrafery thanks for your input. I've checked the injector loom for this in the past and cleaned it but not for quite a while now. It's bizarre because when it's on song now it's impressive. (The best it's ever been in 5.5 years !) I will check these things out and report back. I'm thinking low fuel pump pressure from the pump in the tank, map sensor or lack of boost pressure. I can't understand how it could be boost though. I'm going to put a boost gauge on to check this. Many thanks Nick.

if you had oil in the red plug before, just cleaning it doesnt fix it... you must replace the harness cos the oil makes damage at the connection to the injectors and at the upper plug too

check your intercooler pipes.

Thanks for your replies. I will check these all out and let you know the outcome.
If the injector wiring harness is damaged by the oil would this put a fault code up or put an engine management light on ? There are no miss fires just a lack of power. As i've said at the mo it's pulling harder than ever before ! in all the 5.5 years of ownership.
If i pull up for what ever reason even for just a couple of mins. The power goes back down but usually comes back within a mile or 2 of driving !!!! It's a weird one I can tell you.
The bost gauge will tell me if the turbo boost is all good inc. pipes and intercooler. I'm wondering if the lift pump in the fuel tank is playing up ? Or if the Map sensor is not sensing load or the waste gate is sticking open. Ha ho i'll let you know when i've soughted it cheers Nick.

It may be sticking wastegate which frees up when warm ???