Are there irons plugged in as well? Those consume a lot more power than the machines and exacerbate the problem.
Power droops for extended periods of time will damage motors (the windings will get hot), so it's good that you are looking for solutions.
The reason surge protectors don't work is that they are designed to be sacrificial devices that absorb/redirect a power spike that's only a fraction of a second and prevent power from passing. They don't do any line conditioning.
Be sure the voltage regulator has an internal battery. What you're basically looking for is a UPS (uninterruptible power supply) with voltage regulator capability.