Jerking at steady speeds?

[Darkchase63]

I may have posted in the wrong section I hope I have the right one now?? 

 

I have a 2013 focus 1.6 eco diesel titanium. Bought as a second car to travel to work as company cars were removed at first lockdown. Bought from a local garage who really couldn't have been more helpful. The car has 113000k on the clock and on collection had a few issues the garage were aware of and rectified within a few weeks for example, new fly wheel and clutch due to vibration through steering wheel, new air con pipes, new auto choke due to high diesel usage, plus full service. However I sent it back a few times due to an intermittent jerking when in 2nd at 20mph, 3rd at 28-30 mph and 4th at around 40mph. This happens all the time when my foot is held at those speeds. The garage had it sent to a Ford garage for testing and they came back with a nearly worn out inject 4 which they replaced and seemed to make a difference however this jerking is still occoring in the same conditions and it feels like the engine is choking?? Any advice would be welcome on what possible cause is? 

Thanks guys