running lumpy - mid range revs

[hkpstefan]

Hi - can anyone help?
My surf is running really lumpy at mid range revs. I first noticed it when I got off the motorway on the way home and slowed to around 50 kmh to drive through the area we live in. at around 40 kmh, I was in 4th gear and probably at about 1500 revs, I eased for a corner and started to put my foot down again (not much - not enough to change down a gear) and once the engine was under load it was like it was misfiring. When revs built up again (at around just under 2000 rpm) it returned to normal. Have noticed this more and more since first occured - and is always the same. Ticks over nicely - no missing. on highway around 100 -110 kmh I'm doing just under 3000rpm and it is smooth as anything. Just seems to be mid range revs under load. Any ideas? Thanks for any help you can offer. At this age, the car is probably not worth spending a whole lot on to repair, if there is a quick fix I'd appreciate the advice. Ta.

[dieselboy]

may be dirty induction pipes..? see if they are soaked in oil inside. you will expect to have a little amout of the pipes coated, but if there is loads then this wont help.
also is your air box standard? have you had an aftermarket intercooler fitted or anything?
put a bottle of reddex diesel through your system
hows your water? and engine temp? and oil level/consistancy