Disco 300TDi, odd vibration and lumpy revving at 1500-1800 rpm

Evening all. Got a real strange problem with my D1 300Tdi.

Its been an awesome tool, never let me down and still carry on going but she's unwell.

I run a 50/50 veg oil / derv mix. Never had any problems with it, however this problem is there on diesel and veg.

In neutral, in gear, driving, stopped, it makes no difference.

Rev the truck to 1500 rpm ish and i get a vibration (buzzy vibration) through the gear box and clutch, can feel it in the car. I can't hold the revs there, its lumpy as hell. Give it some more throttle and it will rev past 1800 and then all seems OK.

Its hard to drive normally as your normally around that rev range and its hopping around like mad

Happens if its in neutral, parked up with transfer box also in neutral.

Im thinking fuel pump... or maybe something wrong with the gearbox, getting stiff, i don't know.

Anyone had this before? I don't want to start buying injector pumps unless i need to

I tried changing lift pump, fuel pump, cleaned sediment trap, all made no odds. Lift pump is genuine Delphi unit.

Hope someone can shed some light on this.

Thanks for reading!

Oh, forgot to add - did an oil and filter change the weekend, all OK - checked gearbox fluid level, full.

I would start with simple things first.
Happens whether static or in drive so that rules out a few things ie props, prop joints and rest of drivetrain.
I would be looking at engine and gearbox mounts first, have they got any play in them?

Not that i know of, but then they wouldn't cause it to not rev (which is why I'm thinking injector pump...?)

Isn't 1600rpm the engine speed that the pump does something to bring the turbo in, or increase fuel somewhere.?