my brother inlaws focus has developed a bad missfire

he was giving it some the other day and when he slowed down the car developed the problem, he's changed the plugs and tried a different set of leads just to eliminate the obvious.

any ideas what it could be, when he revs the car it splutters and misses badly and has virtually no power when driving but does seem to be producing boost.

i checked the compression and its fine on all four cylinders and it is not smoking at all.

thers no fault codes stored in the diagnostics but the fault light has flashed on and off a few times but i presume this is because its running so badly

all suggestions welcome.