IMHO the single shaft dual mags (I have one of those too!) are the weakest link of that whole engine. Never skimp on the overhaul intervals, and always use the most reputable overhaul company.
There are two issues with an ignition harness: breakdown of the insulation between the inner conductor and the shield (which can become evident if one does an LOP mag test at altitude) and a problem with the grounding of the shield (which is where the interference would come from).
One needs to be careful doing mag tests at altitude, due to risk of backfire and exhaust damage. When switching to one mag only, be prepared to shut the throttle to idle quite rapidly if a duff plug (rough running) is evident, then switch the mag switch to BOTH, and only then advance the throttle back to the cruise setting.
I had another weird interference issue: the Shadin flowmeter was reading about 7% too high. It took me years to chase it down, and sure enough it was caused by unshielded wiring to the flow sensor turbine, picking up muck from the ignition harness, within the part of the wiring located forward of the firewall. It was an amazingly consistent 7% too... The unshielded wiring was done by one of UK's best known avionics shops, too...