The "flickering" on the Voltwatches seems to be more prevalent when digital servos are used.

But as stated it is normal.

You want to make sure that the overall level indicated stays in the upper green. As suggested, lower than this may indicate that your batteries are not able to supply the required power.

Many people do not realize that a typical standard sized servo can draw up to 2 amps when fully stalled!!!

Bare this in mind, given the capacity of a typical 800mAh flight pack.