New 582 (12hrs!) massive mag drop(s)

After 12hrs running of my new, out the box, 582. Installation in my X'Air was, fine. Prop pitched and mag drops smoooooth. Then it went like this......... 

Originally, yesterday I was jetting the carbs for mixture for EGT and plug burn. - It was running a little cool and plugs a little black. 1 notch on the needles - sorted!

During these EGT runs/flights the mag drops, which had been normal up to now, started to 'cough' a little during the p/f checks -  it was on either mags but not at the same time and not much but then all cleared after running a moment longer to normal drops. 

After a brew to cheer for the clean plugs(!) I went to go for a fly..... Straight away I had 1.5kish+ mag drop, 3500 to 2000ish, on 'no1 mag' and coughing on 'no2 mag'. We had a wiggle check of the external ign wires. Just the same- in fact it got steadily worse till finally at the end of the day, both igns mags had the same massive drops!!! Runs fine with both selected. We put it away!

This is a brand new blue top. I haven't done any wiring checks/invest yet. Any thoughts/obvious fixes/experience of this, pleeeaasse. I will be taking each usual fault isolation/finding steps but any pointers to help would be appreciated.

Thanks!