Commiserations as these intermittent faults are the worst type, not familiar with the Gems system but I have heard of very similar symptoms being cured by renewing the ignition amplifier situated adjacent the coil itself on my 3.9.
Good luck.
2009 4/4 Henrietta 1999 Indigo Blue +8 2009 4/4 Sport Green prev 1993 Connaught Green +8 prev
