2004 Vauxhall Vectra loss of power

hi , i have a 1.9 cdti diesel 150 bhp , i have an intermitent problem when car is driving ok then i have loss of power ,i can't get it over 2000 revs then it kicks back in , don't think it is a fuel problem as i have changed fuel filter and put in injector cleaner but it is still the same , i have read about the ERG valve , do you think this could be the problem , thanks EDDIE