Found it just were you said Steve,not the handest of places i must say ,so toped up now much cooler and seems to have introduced another fan wich i hadent heard before,anyway alls well and thanks again .Rob
i had a 327 chev 40 years ago with similar fault it turned out to be plug leads fitted in wrong holder two sparks going vertualy at the same time and one would draw the other making it miss placed them in wright holder ,fixed ,worth looking at.