lack of power at top end

hi all have a p38 2.5 diesel, starts ok, runs ok, find it is holding back and only going upto 2500 revs in top gear, kicks down ok and will go upto 4500 revs, hates hills unless down hill then seems to rev ok, takes a while to get upto 60 mph, changed fuel filter, turbo pipes, put addative into fuel to try and clean system, have noticed on turning on ignition the normal diesel heater sighn but also an orange pen looking thing with sparks on it is this an injector warning light and if so should i be looking at changing all the injectors, mileometor 217,000 but think the engine has been replaced at some time as no engine number in logbook, please can anybody shed some light on this problem