i have the same problem during races...i even use the RPM ones...
get some brass tubing and slide it over the little flange where the dust boot seats on.
thjis prevents that area from flexing and wont allow the ball to pop out.
failing this...get some FLM ones...and after that you'll be pulling the thread out of the arms
E-revo 3.3 conversion, 249kv outrunner, 6s, MMM
the porthole from the noob world an here has been opened!! that's how i got in.