Don't know to much about modern petrol engines I haven't run one for about 25 years but it sounds as though the 12v supply to the ignition system could be varying. I have never had anything with coil packs but presumably there is a 12v feed which will come via the ignition switch, possibly a fuse and a relay. If you can monitor the supply you can see if it drops when the problem is present.
Hopefully someone will be along shortly who knows more about it and will probably tell me I'm talking out of my hat.
Edited by Peter.N. on 31/12/2011 at 13:22
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