I have a 80" YAK that will not pass ground range check with a Futaba PCM or PPM reciever. I installed a Spektrum AR7000 and it passes range check and then some. Do I need to ne concerned with the RF noise that bothered the PCM/PPM or is the 2.4Ghz make it a moot issue?

i would still try to find it as the 2.4 is only masking a problem.it might get bad enough to cover up 2.4 and then you have been had

will rf from magneto equipped engines be a problem for 2.4?
i have stopped using my magneto engines because of the rf. if i can use them without worries with the 2.4, it will put to use some otherwise reliable engines.

I have a Zenoah G62 in the Yellow Aircraft P47 using PCM and I have no RF problems at all. I would look for a metal to metal contact somewhere in the plane. Make sure the Receiver is wrapped in foam and velcro'd to the airframe as your problem might also be vibration. If this doesn't work try unplugging all of the servos and plugging in one at a time and checking the range each time. It's tedious but necessary. If you have a bad connector going into the receiver that is causing glitches throughout the system it will eventually affect you 2.4 as well.
Goodluck and let us know what you find.