Blimey, gents - it seems ol' Thunderbird has tried just about every which way, yet he's still without his switcher.
T'bird, for what it's worth, what happens if you put things back as they were in beginning, but this time try it using the separate battery again but omit the servo extensions? You may be inadvertanly adding to your woes because it's possible the extensions are acting as antennas and allowing even more unwanted hash to enter the receiver. I'm not sure -- just clutching at straws.
Could it be that your switcher is maybe a duffer?
It's almost impossible to take a stab at the correct answer because there are a myriad reasons what the cause of the problem is. Some Mayhem'ers have access to test equipment. Make the right noises and maybe they will check your switcher over for you.
Failing that, you could always build your own... or purchase a switcher that you know to be a good 'un.
Point your browser at
http://www.action-electronics.co.uk/ and have a word with the veddy nice man there.