Intermittent misfire on 2.8 v6 4motion

Hi, my y reg (2001) 2.8 v6 4motion has developed a misfire. It started by making a pinging noise. By pinging i mean a fairly high pitched noise like someone tapping a high pitch tuning fork. Its roughly every second. This started then about 400 yards down the road it started misfiring and loss of power. RAC took it to a garage. Came up with misfire fault codes on diagnostics check. garage fixed misfire by replacing plugs and leads. On 2nd test drive the pinging noise came back, then developed misfire and power loss. Sometimes it just has the misfire without the pinging tho. Its been suggested a valve guide may have worked loose. Has anyone had a similar problem?

Its a y reg, 2.8 v6 4motion with 93k miles if that helps at all. No warning lights come up on the dash when the pinging and/or the misfire occurs. Only plan of action seems to be for the garage to remove the head to investigate. Garage has admitted they have limited knowledge of the v6 4motion as theyve only done minor work on one before, hence im asking you lt (the experts!)

Help! Many thanks in advance cos i know one of you helpful bunch will suggest something!