Bmw 318 Ci Need Help.

hey, right ill keep it short and sweet so it doesnt confuse anyone,

was having a problem with idle, needle doesnt jump around or anything,just idle abit ruff sometimes, when first experiencing this it was only after the car HAD warmed up it had the idle problem.

changed oil and filter to bmw 5w-30, changed air filter, changed spark plugs,

that improved idle alot,

then yesterday, checkd ccv and pipes, all fine. its an n42, so pipes are solid plastic anyway not the rubber hoses,

put it on diagnostics, cylinder 4 at tickover was reading higher than rest,

so changed the precat o2/lambda sensor, (it is linked to cylinder 1 + 4),

took it for a spin to let car ecu relearn...

after the car had properly heated up no problems!! hurrray, 30 miles of idling fine,

went shopping, all cooled down and the !Removed! idle thing is back but only untill car is warmed up, then nothing?

could that be out to do with the 2nd 02 sensor? after the cat?

the readings on everything else are absolutley fine, no codes or out, just that reading on cylinder 4.