loss of power 2.4 duratorq ldv cnvoy?

Hi have 2.4 duratorq, in ldv convoy drives normal low speed but when on hill m/way 50/60 mph flat out no power after 2000 revs, accelerates ok normal but to pull off with more power just not there, like driving a 1 liter in big van, changed fuel filter and housing, tried fuel cleaner, no joy, no extra black smoke. no white smoke, engine done 186k, oil changed reg, every 6k. been like it 3 weeks, did correct itself once for about 10 miles then went back to loss of power,