Freelander 1 Freelander 2003 1.8 running problem

Hi new to the forum and currently after some advice/help I brought a 2003 Freeland 1.8 a couple of weeks ago was sat in a field for to years previously went straight through mot with only a couple of small advisorys anyway I have been driving it the last couple of weeks with no problems but a coupe of days ago it developed a problem started of as if you was driving and you let of the throttle a bit it would splutter/jolt but now it is all the time while driving I mt will do the same almost like the engine has died then come back to life also if you hokd the rev at 2000 its very up and downi have had the fuel pump off and cleaned as it was very dirty also cleaned the icv but I'm now out of ideas

How's the fuel filter? Was it replaced while the pump was out?