Has it been chipped? That can cause lumpy idle, particularly when cold. Mine is always uneven!
Coolant temp sensor? (I've got a spare one you can have if you want it)
Lambda?
Wouldnt waste your time changing the knock sensor, it wouldnt be causing what you are describing.
My ph1 done it to.
Plugged it in to find Air temp sensor was knackered.
Had some fun times with the boggy from cold though, drive it a lil bit...let off and tap the throttle *pop pop pop* lol