just started doing it completely out of the blue

the car has a 5th injector on it and is only pinking at high boost and
high revs, so i thought that may be 5th were not working, but
its only just been fitted by Stu at MSD and it were fine till wednesday,

i was not going to use the car until it were sorted but went for a drive last
night and pulled up next to an s2 and thought sod it, 'go for it'
and there were no problems, but on way home it did it,
confused

well its getting new injectors on wednesday and is at MSD on the 18th,
i hope its sorted, any one had this problem, ideas to cause would
be great, thanks troops