If so, that could/would cause the sags - also, ask your electrical guy if ANY of the HVAC motors, fans, etc, are "soft start" - this is almost always done with SCR's, and it's the same as putting dimmers on your studio lights
It'd be good if you have a way of checking the line voltage at the wall - best would be a recorder (expensive, unless you can talk the power company into putting one on for a week) - otherwise, you'd need a meter - make a log and check the power a couple dozen times a day and write it down (the voltage, that is) - best if you have exactly the same stuff turned on for each reading, so you know if it's outside the house where the sags are coming from.
As far as I know, there isn't any "magic wand" for finding out what's going on in these cases - just a lot of checks, with the info being run through a mind that's done it before
HTH... Steve