rmh

Don't want to get into a pcontest but a 5 cell batt under extreme load ( relative to radiosetup )should never have gone lower than 5 volts in any case!.
My 6 volt setups under max test loadings on servos in model typicall yrun from a max of say 6.3 to 6.1 under load.
Yourdescripion of crashes describes a typical low voltage failure - which will often after test , a close to nominal level of charge ' what is NOT shown is the severe depression which occurs a few minutes into a flight - for whatever reason - servo loading etc.. This is a basic battery choice issue

I have flown too many different Spektrum setups to believe the failures as you describe them are due to a basic tx /rx problem
Power and reciever setups seem far more likely.