Ka Missing The Odd Beat

Hi all

as you no doubt read in my previous thread I have been sorting out a friends KA, it has always had a slight misfire but only under load, even after changing Lambda sensors, plugs look ok.

I am told last year it has had plugs coil & leads which helped a lot but did not totally cure it Hence its still there. Was also told it had a recon head as a spark plug snapped while changing them

Planning on checking for error codes, the engine management light has never been on as far as I know, but there may be codes stored.

Apart from coil, leads & plugs what else could cause and ignition problem that I could check?

KA 1.3 03 plate 55,000 miles

(duratec engine)

used daily driven 'quickly' mainly short journeys but the odd long run every now and then, Have used redex injection cleaner a few times recently