msd digital 6 plus

[tuffxf]

G,day,
I have a sbf powered bracket car with a 6al, ran finefor 6 yrs, hundredss of passes.
Bought a digital 6 plus to step it up a bit, used the correct coil, used the optional shielded trigger wire, had 14.4v supply volts, zero resistance on the earth wire back to the batt neg terminal.
All the wiring was as seperated as posible from each other.
With a msd tester it worked fine up the rpm limit which was very close to where it was set.
Went to largest meeting of the year, first pass misfiring in the burnout and at the top of each gear from about 5000 rpm up.
Fattened the carb, lifted the rev limiter, nothing made any difference, a mate had an old 6al, threw it in and run perfect, still in there.
Sent the msd back to the agent here in Aust, tested it ok, sent to another place ok?
It honestly beats me! the batt is in the boot, the ignition was not wired all the way back to the battery but positive went to the hot stud on the starter solenoid, this is fed with big dia welding cable, earth wire went to the frame, batt neg in the boot is earthed to the frame, maybe not correct? can't detect any voltage drop or high resistance but.
Maybe going back to the batt would shield the unit from picking up interference with the engine running, i believe that it is picking up interference when the eng is running? not sure.
There is a pigtail of unshielded cable from the pigtail in the dizzy?
If you check the ac output out of the alternator it is near perfect so it beats me!
I wondered if the pickup in the dizzy is not suitable as it is a ford bosch one, but the engine ran fine on an engine dyno with a digital 7 ign and exactly the same dizzy and cap and leads, so rotor phasing shouldn't be an issue?
They seem to be very finicky, i went to the digital to try to mirror the magnetic compensation circuitry of the digital 7 and keep the timing identical up top as i heard that the analog msd's retard slightly at high rpm.
Anyone have any ideas or experience with these things?
Thanks very much!
Paul