Running lumpy

[Juddy]

Hi guys & girls, my megane 2.0 petrol 55 plate is running lumpy and cutting out unless i keep on the revs, on the dash it says check injectors. While doing this i changed all 4 plugs and pencil coils just for good measure. After removing the pencil coils i noticed No 2, 3 and 4 was a slight yellowish colour (after reading various threads i believe this is a good sign) but No 1 wasn't. Anyway after changing no 1 injector the car seems to be ticking over ok but on revving it it still sounds lumpy and after using a code reader it's coming up p0300 (random/ multiple cylinder misfire detected). Took all injectors out and turned car over while spraying them into a container and all 4 are good. I did how ever notice that on the side of the engine r/h/s there's a round black plastic cover and after spraying soapy liquid around looking for leaks this found to be leaking? Could this be the problem? Thanks.