What is causing it?

I am running MMM combos in everything and powering with 4s lipo.
And a Spektrum Dx3.0 and SR3000 tx/rx.

I try to run a Hitec 7955tg and i need a Spektrum capacitor to avoid it being glitchy.
I run a traxxas 2075 servo and it is perfect.

What is casuing the glitch?

Im asking because im getting ready to replace my Radio system to allow for more models. But i would like to eliminiate this issue if possible. Caps are cheap if i have to i will run one on everything and call it a day, but if there is a more "proper" fix i would like to know.

The glitching is caused by the current draw of your servo relative to what your ESC and receiver can provide.

MM's stuff = known to be glitchy w/o a BEC and external or cap
Spektrums stuff = known to be glitchy w/o an external BEC or cap
They just don't provide a good, constant BEC to the servo.

The real miracle is that with those combination of products, you have a servo that works without anything added in.

There's no harm in running the cap, especially since most good servos tend to glitch off the MM especially, you'd be looking at a new ESC as well.