Help...anyone out there who can advise?

During a trip away with the family over the easter weekend, my car suddenly started to lose power (boost) when excellerating. It appeared that the car went into limp mode and i was unable to do more than 80 mph on the motorway and the car was very sluggish when pulling away. Power came in and out and it was difficult to drive.

Having sought some advice, i checked the MAF sensor and replaced this with a new one from GSF. However this still did not solve the problem.
Does anyone have any ideas what is causing this?

A mate brought along his diagnostic equipment and found that the maf worked at idle but as soon as we pulled away and the boost increased, the car lost power again as if it had gone into limp mode again.

Any ideas? Please... (car has done 78k miles)

Thanks.