The plug boot looks fine. If the plug boot slips off the plug while the engine is running, the ignition gets fried.

A poor range check could have several causes. Are your ignition components (including battery and switch) at least 12 inches away from all receiver components (batteries, switches, servo leads)? Have you eliminated all sources of metal to metal vibration - metal clevises on metal threads need locknuts. Any metal parts touching other metal parts have to be tight - any wiggle room can cause interference.