intermittent faults - HELP!

Right this should get your thinking caps going. 

Right i'm having problems with my car. i'm having a intermittent misfire. when it happens i'm normally doing 50-60mph uphill. sometime its fine and sometimes it'll lose slight power, juddering, and can hear a cluster of clicking noises for a few seconds, if not the engine light come on. i had new spark plugs which cylinder 3 didn't last a year and was in a bad condition and stopped sparking. coil packs are all brand new as well  a month later i broke down as misfire cylinder 3 came up but managed to get home after turning the car off and on. took it to the garage and they replaced the spark plug (free of charge) they said see how that goes and bring it back if it continues and we try something different. unfortunately when they do the test drive to get it to do its thing sadly the girl behaves. few days after the it had that spark plug changed. i started the car up and the engine light came on. i plugged in my code reader to get p0351 - powertrain ignition coil A (primary/Secondary circuit). took it back to garage and the bloody light when off. the looked at it and said light didn't come on when we had it but she drives perfectly and they were confused as if that fault was coming up it should be mis firing. they said it could be the ecu but weren't sure. 

any advice would really help.

Cheers