I'm taking a 6V 500ma power supply and feeding it through a 7805
That doesn't sound good - your servo wants 6 volts and you're reducing it from 6 via a voltage regulator. I suspect you're not even getting 5V. Your wallwart can provide 500mA, the general rule of thumb I've seen here recommends an amp per servo. In short therefore, your issue is likely power related - see if you can scavenge up a more hefty supply.