1. Plug like Matt said, the resistors do go bad in them, esspecially Champions!
2. Check for water and corrosion in your servo's, reciever and connections, any green spots replace.
3. If your servo's aren't in the radio box they need periodic maintanance, and resealing so they don't get wet.
AM radio's do get interference from electrical noise or metal to metal rubbing. (any brand) FM is better in a gas boat, PCM even better and the NEW 2.4 ghz are proving to be good too, beware though the first 2.4 ghz radios out were bad, lots of range issues.
How far away was your boat when you had problems? Do you know how to range check your radio? What kind and how many batteries are you running in your boat?
Please don't let people swim while your running, especially if your having radio problems.