V4 firmware has been updated to 4.01 and 4.02 as 4.0 was found to have several bugs... so make sure you are not running 4.0.... the beep can cause the prop to twitch sometimes as the tones are coming form the motor... Try 4.02 and 3.27 or 3.28
If the problem is still there the magnets in the motor may have changed ... I had an airplane motor that de-magged and it got the twitch a while back but your motor could be fine.
If you want to have the ESC checked you can send it in to Castle via the online form...