You could have one or several problems. You could have loose or bad connections anywhere from the pole through your panel to a single circuit. Loose or bad connections can have many symptoms, many of which you've already stated and can be very dangerous if not corrected.

You stated you replaced some breakers, did you personally replace the breakers or did an electrician replace them with the intent of solving the problem?

I would contact an electrician or the power company to check out the problem.