It's likely that the impedance of the battery is rising, affecting its ability to supply bursts oh high current to operate the motors, dropping the voltage below the required level. This happens before voltage drops too far at low drain currents - when you typically check you battery indicator.
So get fresh ones, and charge them once the first bar is off. There have been some recent post on discharge and thresholds, like
this one on the German pentaxians forum, and the elaborate
article by c.a.m. here, both indicating that the first bar covers about 50% of the capacity - for original Pentax batteries.