Are you noticing this with certain appliances running?
The reason I am asking is do you notice this happen when certain loads are being applied. Can you possibly (and safely) check to make sure your neutral is fastened correct and not loose. However if you are not familiar with electricity, please do not do this.
An open/loose neutral can cause funky things to happen, especially when loads are applied.
Check this link out, and then click on the "open neutral" tab