have not seen it on nitro cars but it is common on the electrics running savox servos with spektrum to have troubles.with electric cars it depends on the bec out put if you experiance brown outs or not. suprised that would be the trouble running a seperate battery pack
add a cap to the rx.spektrum make one for it (glitch buster) or you can make your own
most electronic shops will have a 4700uf 10v cap for less than $2
just solder a old servo/extension plug (pos/neg) on to the pins of the cap and plug in to receiver.
i have had a few savox servos and really liked them,but i have a mid size one in a pan car and it seems to loose its centre,i emailed savox but never got a reply