The only possible cause I can think of is the rudder/steering wheel assembly. There is aluminum to steel (which was assembled according to the ARF instructions). Is there any possibility of aluminum to steel causing interference?
Short answer: If two metal parts can move relative to each other to make-and-break contact, yes it
may cause interference.
It's also possible that something else is causing your problem.
Some
wild guesses:
Try routing the Rx antenna at an angle away from the fuselage (e.g. to top of vertical fin, or to tip of stab). It's best to avoid positioning the Rx antenna parallel to any other long wires.
If you have long (>10 inches), servo-extension wires, use wire-sets that are twisted together. Twisting the wires together can reduce EMI.
Tx antenna screwed on tight?
Clean off any grease, oil or debris on the collapsible antenna that might be causing intermittent electrical contract between the sliding antenna-sections.
Does your Tx have servo-reversing switches? If yes, you might try cycling the switches a few times to clean the contacts.
Bad/dirty Rx switch, or cracked wire in switch harness-wiring. (happens often)
Cracked/intermittent connection inside battery.
Good luck, hope you find the solution.