2005 Citroen C8 suspect turbo

Driving on motorway yesterday, was at roughly 70mph and switched on cruise control, speed and revs dropped dramatically, then struggled to get above 55mph. Pulled over for an hr and then back on road again. Car was struggling to get up to 70mph, revs were sitting at 2.5k, even when I floored it revs barely went above 3.5k and speed didn't alter above 70ish. This was ok until I hit a med incline, the revs then dropped to just under 2k and speed to 55ish, pushed accelerator but same as before revs increase to 3.5k but no real difference to speed, occassionally the car would momentarily speed up then drop off, as if turbo was trying to kick in. This stayed the same for the next 2 hundred miles, 70-75 on downhill & flat sections, but dropping to 50-60 as soon as incline reached. Then all of a sudden turbo started working again, I was able to put cruise control and vary the set speed up and down with Turbo responding as expected. Ran ok for 30-40 mins then back to original problem, a further 1hr down the road the turbo started working again for another 30min.
Not the most clued up in relation to Diesels & Turbos so any advice would be appreciated.