Well, at least here are two ways to investigate:

1. Ask the local EMC (electro-magnetic-compatibility) Company, the one who takes case for all kind of intereferences checkings, and it is part of the Goverment, and maybe of the Police, to come with their
equipment and measure the field. You are paying Taxes I believe in the country, so you are then well
covered by their Services, as wtitten on some of their cars, "To Serve and to Protect". They might do
that if you find appropriate way to ask and a bit friendly challenge them with such "parasitic" emission
which has gone "out of control", maybe ... well I am not in USA, so you should know better what you
can or cannot ask them for. Well, make sure that they do not close the Airfield as a "Public Danger".

2. Put on Air hellium filled in baloon like those which are sold on the public events, your receiver
with all servos and connect the servos to some flaps and ask you friend to walk there where the
crashes happened. I.e. the baloon will be the "flying RC" and then you and he watch if those
flaps refuse to follow your Transmitter ... This way you can go to up to even 50 high and see
what is there, if "anything" is there... At least this might be fun for you and your friends. If
you make it, please post some photos on RCU so we can have fun here too and spread the
Experiment arround. At the end this is lot more easy than to build and ... crash another plane.

Well, at the end, those new 2.4GHz systems which are coming might solve this problem or even
make it lot worse ... Anyway, fail-safe/save options in any king of Radio are good thing.
Especially for Gas-engines, 2.4GHz systems are inherently lot better shielded against most of
the intereferneces but the problems with them beside the wave-propagations one is that they
are in "baby-age" and suffer from "baby-ilnesses" and it might take 1-2 years more to get
them well in place and maybe replace the current FM radios.