OK -- if this happens every time you loosen the cable, then at a guess, you're not actually seeing an E-field signal, but are rather viewing the effect of loosening the cable (either one should produce a signal that would be amplified by the amplifier board).
That leads me to guess that the amp board is probably OK, as it's amplifying what it is receiving, and since DC is getting through to the preamp board, the cable is probably OK, too. Whatever problem you're having is likely on the pre-amp board itself. But at this point, I'd probably attack the preamp with a mulitimeter and double-check parts placement/soldering, and if there's nothing found check the amplifier board.
