I borrowed a 50 ohm dummy load thing to put on the antennae end of the coax, no engine trouble on transmit, so apparently this confirmed problem is RF radiation interference from antennae and cabling all good.(antennae is about 3m from panel but apparently were to expect more of this as radios keep getting more powerful).
Added Tracy's fix to EC2 (got the only .1uF cap in our local hobby shop).
started engine, transmitted, engine perfect. so I'm saying its fixed.
So thankyou to everyone that helped with suggestions. I still don't quite understand why this problem took so long to surface, I have 9.5 hrs ground running time on the hobbs & 4min flight time, guess it must have only become noticeable as the tune got better. also for most of the time I've been fixated on fixing temps.
Will give the plane a very thorough pre-flight tomorrow arvo for hopefully a longer flight on Saturday.