hi Guys....after some advice here

Ive been feeling a very slight shudder from under my drivers seat and up & down the centre console. Initially i thought it was something to do with the gearbox & diffs as ive just had all gearbox+filter and front+rear diffs oil changed. The shudder is only felt in Park and occasionally in Drive (with my foot on the brakes at the lights). So off i went, back to my mechanic for a quick look over. On the way i had a clear stretch of road and put my foot down to see if i would get the vibration throughout the gear changes. At approx 3000rpm the engine management light started to flash for a few seconds before staying on permanently. Fortunately i was at the mechanic within a few minutes of the engine light initially flashing. Diagnostic error codes have come up with P0304(cylinder 4 misfire) and P1300(random multiple misfires). On a visual inspection (without taking off the rocker cover & spark plug+coil covers) the engine block is absolutely stable, no unusual sounds, idles normally in Park at approx 500rpm, gearbox is very smooth throughout the changes, exhaust gases are transparent (no blue/white/black smoke). However, the drivers side exhaust does make a slight 'putt putt putt' sound at the same time as when the shudder occurs.

The mechanic cleared the codes and we took it for a drive. Exactly the same symptoms as before. Drives as normal and shudders in Park and in Drive at a stop. Take it to 3000rpm and the engine light flashes very briefly before staying on permanently. Same error codes again, P0304 and P1300. Mechanic cant work on it until the weekend so its been parked up.

Im just looking for a methodical checklist of items to look at to determine the cause. So far ive got:

-check for fouled spark plugs & oil in spark plug well,
-check individual leads/boots/ignition coils
-check PCV hoses

Anything else ??

Just solved this..mines was simply a fouled spark plug and bad coil pack. Hopefully yours is as easy to do

UPDATE :

After replacing the spark plug and coil on cylinder 4 the issue has been totally resolved.

The spark plug on cylinder 4 was very slightly fouled with oil deep down in the well.....literally a millimetre (of oil) in depth from where the spark plug goes through the hole. Every other plug is bone dry and there is no breach of oil through any of the cam gaskets. However there is a very small oil deposit on the silver clip that holds down the coil (on cylinder 3). So am really confused as to where the oil has come from. Could it be oil vapour deposits and/or oil travelling up the spark plug from the cylinder side below?