C230K (W203) misfiring / idling problem

[thefootster]

My 2003 c230k has developed a misfiring problem when it is idling. Basically the engine management light comes on and it goes into limp home mode after no more than a couple of mins idling in traffic or at lights etc.
had a diagnostic done yesterday and it shows a misfire on 3. Changed over the coil and plugs but it still shows as a misfire on 3. The cabling that runs to the right of the engine and on to the control unit was then checked and the control unit reprogrammed but that didn't work either. Then the control unit was replaced which seemed to work for a bit. It still seemed to be struggling to keep resv but the management light didn't come on when idling. Then I drove it tonight for 100miles on the motorway and the management light came on at the 1st set of lights I stopped at. Meanwhile it has just coat me a fortune! Any ideas?

 

[WG M-B]

Sounds like valve guides

 

[thefootster]

Sounds like valve guides

What are valve guides? I'm not sure if it is relevant but it had the chain and valves replaced 2yrs ago after the chain snapped.