2000 Transit 2.5 turbo

[RichardSys]

We have a W reg 2000 motorhome based on a 2.5 turbo tranny with only 55,000 miles.
All is fine and it runs and starts well but....whilst cruising along around 55mph with steady pressure on the gas pedal, it suddenly loses power as if you've taken your foot off the pedal. Apply a bit more pressure and its fine.
I thought this might be a throttle position error, but, whilst you lose power, if the load on the engine is reduced such as when you do over the brow of a hill, the power suddenly comes back it suggesting its a vacuum issue or something. No codes showing, no obvious leaks in the induction system, all vacuum hoses appear fine so I'm at a loss. Any suggestions ?