Have you tried servicing the panel? I.E tightening down the lugs on the incoming mains? Sometimes the neutral is loose on the POCO side, but I've corrected the problem in the main panel as well.
Don't take crap from the POCO. Document every conversation and call that you have made to them and the names (first and last) of the people you talked to, as well as the exact times. That way, when things start blowing up (and they will, if they already haven't), the POCO is liable because you notified them of the problem and they failed to act.
Personally, I have never seen a POCO drag their feet on a loose neutral problem. They should be well aware of the danger involved as well as the havoc that is wreaked by such a fault.
As far as rigging up a customer-side solution, it's ill advised. The problem needs to be resolved.
Good luck!
