Is the power source to test it a regulated power supply or battery? Monitor the voltage at receive, receive with squelch open and during transmit. In our Dittel portable field/mobile unit with a charger and internal nicads, it behaved similarly until both the regulator chip and nicads were replaced. Nothing to do with the squelch setting except that tiny change in milli amps draw. Assume you already looked at the tx signal "clipping" on comm monitor. Good luck. Hope its something simple.