Originally Posted by
mgmidget
change plugs and leads first fella, see if its still thereafter changed them if its not bonus if it is change the pack but clean up all the earth wires and the cradle what the pack sits on as the connection for the earth is underneath the pack where the bolts go thru

Cheers mate, I swapped the HT leads over from the other (written off) focus I have at the moment, I noticed it didnt do it at all after that, I swapped it back to be sure, and still can't notice it.
One thing that I did notice, the battery was a 063 when it's meant to be a 075 size. Had it tested, holding 120A out of it's rated 300A. Swapped with the other focus battery, 075 holding 380/500 and it's been fine since.
Just got some replacement headlight units to fit now, current ones are pissing in water.