Considering all the reports of glitching with power hungry digital servos on Castle ESC's and the fact that the problem appeared with the installation of a new digital servo I would start looking as that as your cause.

The two most common solutions are to use a glitch buster (Capacitor plugged into the Rx across the + and - pins) or to use an external BEC.

If you're servo can handle it, many people have also reported success providing the servo power directly from the battery.

You could also change the servo, the Hitec 985mg is often said to work well in Castle ESC's but only use it if it's powerful enough for your application.

Another solution I've only seen mentioned once or twice is to turn the BEC up on the MM. I believe it's only 5.5V stock, if you push it up to 6V maybe that will help you out

Keep in mind most often the problems are reported with the MMP and not the MM although some people claim it affects both ESC's. As I said before this is my instinctive answer because you said the problem occured after putting in your new servo.

I hope this helps in some way, good luck.

Star