While this solution may be a bit extreme (a couple of thousand dollars IIRC - call them), you might have some luck with a switched power supply such as this one from Interpower.

It takes the incoming AC voltage, uses a switching device to create high-voltage DC and then re-creates an AC waveform via PWM. We use them for creating 230V/50Hz power off of our 115V/60Hz lines for AC motor testing.

In your case you'd set the input and output to 115V/60Hz. We used this device to help with a spate of dirty power (which made our motor speeds vary) with good results.

If it didn't solve the problem you could probably return it.

-Zeke