help.300tdi wont rev

hi hope someone might be able to help me. i have a 1998 300 tdi disco that struggles to get to 2000 rpm,i have changed all the filters and checked the lift pump but still no joy. the injector pump is not flyby wire but does have three wires going to it[not sure what they are for]the alarm has been playing up for a while now could it be that.any help would be very kind

Hi It could be a faulty shut off switch on the pump i had trouble like that on my fourtrak it was only letting through half the fuel changed it for a new one and solved it, I asume the 300tdi has one,will have to look at mine, worth alook.