It sounds very much like it is blowing the spark out at high boost.
THIS IS NORMAL, as ford fitted such crappy coils to them as STANDARD, ie the only good place for it is the bin.
I used 909 leads and they caused running problems, so switched to MAGNECOR, massive difference much better.
I then was still not happy as it had a few misfires when cold so on went a GROUP A coil, this sorted it.
It also improved bottom end response enromously ran smoother all the time, and is the bset £32 I ever spent.
These firt dstraight on once you have replaced connectors on the ends of the (LT) wires as they use spade connectors and not the round one on the oe coil.
What gaps are you running ?, 0.6mm works best on std coil, you can open gaps up on group a coil as it is not weedy like the original one.
tabetha
ps timing belt has nothing to do with this