With my Spektrum I tried the cap and then also tried a lipo receiver pack with a 6.4 volt regulator and still had problems with my Spektrum going into fail safe. Took out the Spektrum and put in a good reliable synthesized setup and no more problems and everything works perfect.
Alot of guys have problems with Spektrum and brushless systems. I know I did with two brands of radios and btoh Pro versions and standard version Spektrum and also two different modules in two different radios.
I have no switched to a new Futaba 3PK Super in the synthesized version and no more problems. It seems the new 2.4 from Futaba fairs better than Spektrum but at least with synthesized I feel much better. Plus I still have 30 channels to pick from and with all those Spektrums and 2.4 systems I never have to even worry about frequency problems.