If you are using a FLEX-6700, FLEX-6500 or a FLEX-6300 that using an 8-pin Foster connector as the mic connector, you have a mic connected to it and you are experiencing the active slice receiver tunning up or down on its own, then there are three reasons why this might happen.
1,) The mic connector is not properly inserted in the 8-pin Foster connector. Heil mic cables have a history of the mic connector being a little smaller than the 8-pin Foster mic jack where the mic connector "key slot" does not prevent the connector from being inserted improperly. Inserting the connector improperly and being off by one pin or two can result in the PTT circuit grounding the step pin. Please check that the mic connector is properly connected but visually looking at the connector key slot and the 8-pin Foster moc connector key "ridge" and making sure they are aligned correctly.
2.) A wire in the mic cable has become frayed and when pressing PTT (pin 6) it grounds the up (pin 1) or down (pin 3) lines. If you have an ohmmeter, you can check the continuity of the cable. For a complete description of the pins, see the FLEX-6000 Hardware Reference Manual on page 18.
3.) All of the lines in the 8-pin Foster mic connector are protected with a small ESD protection chip. It is possible that one or more of the chips are blown and that this is causing the behavior you are experiencing.
It is recommended to start troubleshooting by using the hand mic that came with the radio which should not allow the mic connector to be incorrectly inserted into the 8-pin Foster connector. If that works properly then you need to check items 1 and 2, because this test would eliminate # 3 as a possibility.