P0400 refers to the EGR valve which, if faulty, can cause the misfire.
You could either remove, clean and refit the EGR or buy a new one. If buying new make sure you buy a good quality branded part, some of the cheap copies are rubbish.
It is possible to blank off the EGR using a blanking plate but this will probably put the engine management light on.
frank
March 2012
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