Grounded Neutral Voltage Fluctuatons
You have a totally different problem!
Your root problem is a lost neutral. Probably somewhere up on the service drop to the pole, the neutral wire has parted.  Happens all the time; neutral is also the carrier wire that carries the weight of the other wires.
So you should check the panels for neutral integrity just in case it's something obvious. If you find nothing,  it's probably the pole line - call the power company out and have them fix their wire.
Neutral is NOT dependent on any grounding.  The power company supplies hot(s) and neutral from the transformer.  That is all you need for power to work properly.  An AC power system will work with no grounds at all if things are in order.  The only reason we do anything with grounding is to increase occupant safety.
So what's the deal with this HUGE red herring??
Well, when you lose the neutral wire, neutral current can find an alternate route through the Grounding Electrode System.  It's not a very good route since it involves traveling across dirt, but it's better than nothing and will provide some functionality.   The route is:
The panel's neutral bar... via the main panel's Neutral-Ground Equipontential Bond ... to the Ground bar ... to the Grounding Electrode System (GES)... And I'm betting your neighborhood has metal water pipes... to the Grounding Electrode System of a neighbor... through their N-G Equipotential Bond.... to their neutral bar... to their wiring... back to the transformer.
This is a terrible state of affairs.  You are not only moving current on your GES, which is a big no-no, but on your neighbor's grounding as well. Their neutral service wire is carrying not only their neutral current, but yours as well!
We don't ever want current moving on the grounding wires.  Those are only for use during transient ground faults. Normal service currents must be all on the neutral, the wire intended for that.
Remember, the grounding systems are only an &quot;optional&quot; safety bolt-on; power will work without it.
Your power will not work without safety ground, which is how we recognize a Lost Neutral.
Barking up the wrong tree
Your &quot;electrician&quot; did not actually care about having a working neutral wire.   Your electrician was trying to restore the original failure -- where neutral is out, but ground wires and water pipes carry your neutral current to your neighbor's.  Shame on him!
Now, fix your neutral.  Contact the power company and tell them power is out and have them fix their wires.  This is done for free.  And it takes an hour not a week.