Recently been flying 757 200, with sim throttles I cannot get them to match, one of them moves somewhere else all the time, either one will always be different.  I have external throttles and they are matched through FSUIPC.  How can I get the screen ones to match as even with autothrottle they are different and the only way I can get them to equalise is using the mouse but then seconds or minutes later one of them moves.  I cannot see anything in setup.