A couple of years ago I bought aJapaneseILSAN Extra 300L ARTF. Very nice kit, but when I flew it, it would at times be uncontrollable for a few seconds and after frantically moving the Tx sticks, it will regain control and flew normally. After a number of these adrenalinencounters I went back to the hobby shop for advice. They told me that a number of those ILSAN kits had crashed - the cause being that the planes were factory covered in some "printedmetallic" foil, and that the 35 MHz aerial needs to be routed outside the fuse.

This did not solve theproblem and I decided to re-cover the plane in plain Oracover. It did not solve the problem either.

Thekit had a pre-slotted hole for the 2 aileron servos to sit side by side in the centre ofthe wing. The elevator and rudder servo slots were situated underneaththe "cockpit floor" above the 2 aileron servos (upside down above them). On close scrunity I discovered by chance that when Ifit the wing to the fuselage,the aileron,elevator and rudderservohorns lightly interlockedwhen the plane wasin abanking climb. They interlocked and the frantic movement of the sticks unjammed them again.
It was easier to move the elevator andrudder servos forward 40mmso they are not above the aileron servos anymore and the problem was solved.

I was lucky - in this case, with new JR 35 MHz PCM radio gear,it was not the sticky"metallic type" foil covering causing the problem - but the kit manufacturer that situated the servo positions wrong.This was my experience with "radio interference".

A few months ago I build a Pizza Box Flyer - same thing - it will fly nicely and all of a sudden make uncontrollable loops and turns and then regain control again. I tracedthe problem to a brand newswitch harness power lead to the receiver that did not made good contact in the switch-I replaced it and problem was solved.
So I agree with NM2Kthat there can be a number ofcauses.

I'm planningto do some 30 - 100m range tests with 35MHz and 2.4 GHz gearin about 3 weeks time - and do1 test with aluminium foil as well -aka Mythbusters style - just to see whathappens. Until then