My unprofessional opinion, if everything but one light was off and you have fluctuations, and everything from the meter to the panel is tight, it could be the transformer feeding your house, the drop from the transformer to the meter, or you're at the end of a long country road and the power to the house fluctuates. There may be nothing you can do about it but complain. I think I'd call the power company, explain what's happening, and see what they say.
Depending on where you live, they may not have too many crews available, as those from a huge number or states have sent crews to New England to try to restore power to people that have been wtihout since last week's ice storm. So, you may not get anyone to come look for awhile.
That voltage dip is visible, but probably not dangerous (it is annoying, though). I'd put good quality surge suppressors with line filtering on any expensive electronics and maybe consider a UPS if it is the incoming power.