If it also has an internal mic thats turned off when you plug the external one in, for which you have to turn the boost on to work, then likely whats happening is that the internal mic would be over sensitive and feedback hard when you unplug the external one since the boost is on. Don't see a way out unless you make sure you turn off the boost or turn down/mute the mic before unplugging out the external one.
R.I.P Rangeral, To one of HWC's best moderators and a great guy
By the way, what does BTW stand for?
It is better to be tried by 12, than carried by 6.