what radio/rx and servo are you using?
my bet is the one you sent back is fine. no issue with it at all.
I have had many issues because the BEC is not strong enough to deal with Spektrum, KO digital servo and a PT along with a hot brushed or LRP only BL motor. No issue with the new orion BL motor or the novak BL motor
Glitching at low speed and no response under full throttle.
There are a number of fixes.
On my TC edition I have added an extra power cap to the spare rx port and this solved the problem.
On the 2007 in my 2WD I have run the blue fet wire to the +ve on a spare slot on the rx to solve the glitching.
alternatively change to JR or Airtronics servo and this solves the problem too.
If you are using a different setup to mine then I cant really comment but I would go with another power cap first.
It seems that the LRP motors draw just a bit more current than the Orion BL or Novak BL I have tried and the power cap solved the problem.
Another twisted way I was going to fix it was to go back to my KO FM module as I have no issues at all with that installed
Ben