Zd30 engine missing at 1800 rpm

Hi all i hope someone may be able to shed some light on this issue
I have a dec 2000 zd30 which has performed faultlessly until recently
The engine starts and idles as per normal, when it is driven it starts missing at about 1800rpm, change up a gear and once it is fine until 1800rpm and so on.
I had the diagnostic errors read and the only 1 that showed up was caused by me from when I had removed the maf plug.
I changed the fuel filter, slightly better maybe
I bypassed the fuel filter and lift pump, this appeared to fix the prob so I fitted a new lift pump, test drove it and the prob appeared gone.
The next day I went for test drive, after about 40km highway driving the missing reaccured.
Replaced the flexable fuel line to and from the fuel filter in case air was sucking pass the old hose clips, no better
Replaced the MAF sensor as the voltages were not correct on the old unit, this seems to have made a difference to the engine, appears to run much smoother but has not fixed the engine missing.
I feel the prob is either lack of fuel or air in the fuel line.
Hopely someone has seen or had these symptoms before and can help

Cheers jim

I'm reading lots of reports of the primers on the fuel filters leaking atm..
When pressing on the primer plunger does it become firm after 4-6 pumps?
I'm thinking it's drawing in air threw the plunger and into the fuel line..

Well after wasting a couple weeks of my holidays trying to trace this problem all it took was a 5 minute conversation with a desiel mechanic today to sort the issue, so far.
After telling him what I had done so far he asked if I had replaced the seal between the filter and the lift pump on the lift pump thread. I said I had put the original oring back where it came from, only to be told it should be a square section seal not an oring.
Without the correct seal air is sucked into the fuel stream.
Only good that came from this was replaceing the MAF which failed the voltage tests, it has breathed new life into the patrol. The cook is even impressed with the difference, we obviously got used to the gradual loss of performance.
Thanks Jim

Did you read my post above??