2008 trafic 115dci?

help needed please, took van for engine service before a planned holiday After an engine service oil, oil filter, air and fuel filters, the van intermittently cut out whilst driving just for a second then all ok again also with a slight lack of power at low revs then engine management light came on once while driving but went out.
Apart from the intermittent cut-outs it drove ok, I took back to garage plugged in to their computer; the mechanic took fuel filter off and away and refitted but no change.
Filled with 67L of diesel on Tuesday the 3rd at Tesco Mereway and within a mile of the van started to miss at around 50mph then developed a low rev missing/ lumpy idle. Went back to garage they couldn’t look at it, so on Wednesday I called AA out, no faults were showing on computer and checked I had not put petrol in the diesel looked in filter and checked receipt
Thursday morning on way to town the red engine management light and the stop light on dash came on every time the van was at idle but goes out immediately when pulling away.
Went back to garage on Thursday eve and they fitted another fuel filter and commented that air bubbles were present in the fuel line between filter and high pressure pump the engine management and stop lights come on almost every time the van is at idle and flash sometimes.
2 weeks in the garage to replace one injector No4 and a bill for £1000.00 picked it up lumpy idle all good now but within 3 miles the intermittent lack of power and split second it loses around 1500 rpm and the red engine management and stop light come on then go out, took it back to garage they diagnose wiring fault on injector 3 then put it down to something they may have done and tell me its fixed pick it up next day and its no different from before