Kango 1.5 dci 55 reg Loss of Power while driving ?

I have 55 reg kango 1.5 dci
it seems to loose power when driving starts first time every time not a problem but after a few miles of driving I start to loose power I change gear and still the same I take foot off the throttle and apply again its still the same I drain fuel tank in fear of contamination ,change fuel filter I checked the EGR valve that's fine but still give it a clean, check the air filter...had it in the garage they cannot find any thing wrong at the cost of £250. is there any thing I should do because this is driving me nuts.......