Spektrum/Tekin/Savox brown outs are a known issue. Even in 12th scale with low loads most people have given up on this system and go with Futaba or Sanwa 2.4 systems for reliability with Hobbywing and LRP speedos. (Note, in 12th we don't use Savox because the output spindle is in the wrong place.)
Spektrum receivers, Savox servos and Tekin speedos are involved in too many of these enquiries for it to be a co-incidence. With the tolerances on the electronic components in the servo, receiver and speedo, it is entirely possible that some set-ups get little or no problems, and others have or develop problems. If you trawl the forums you will find a lot of enquiries of this type that mention Savox, Spektrum and Tekin. I've yet to see any involving Futaba, LRP, Hobbywing or Sanwa.
By all means try the glitch-buster capacitor in the receiver, but for a true diagnosis you need to attach all your stuff to a Futaba or Sanwa 2.4 system and see what happens. I am not dissing any of these products (used them all myself at one time, except the Savox) just letting you know that the combo you have is often cited by users on forums as having these issues... eventually. HTH
