the Mercury is not unstable- the Jeti receiver seems to make a bad UDI signal when it is in 24 channels mode. A UDI24 protokoll is not existing!
Again: It doesn´t help you that you have spare channels on the receiver: it is not allowed to connect servos here! The Power supply to the receivers is fused to 500mA. If you connect servos here, the receiver will go off!
The only thing which will make your setup work is a Royal SRS. You go in with UDI 16, then expand the output functions by servo matching.