The problem you have is simple but its in your tank so no one will find it!!
There is a little mesh filter on the fuel pump.This gets blocked with tank grime but usually clears itself when the petrol its sat in washes around it causing an intermittant problem.
What happens is when its blocked or very grimey it causes a drop in fuel rate to the pressure reg-the chrome ufo thingy near the lh side of the cam cover-(focus). This in turn causes a loss of pressure at the fuel rail and spluttering under load/bad idle.
Only cure it seems is to take it out and clean-redex awesome for fuel line etc but wont clean this mesh-ive tried!! Best of luck,
DUDOCO.
P.s. you may have no fault codes showing.Its an arse of a job but itl save you a fortune at the garage! Trouble is if they dont see a code they dont see a fault at times ECUs!!!!.