Some outlets are dead, others fine

A few outlets throughout the house will go dead for a few minutes to as long as a day or two, and then come back on. It doesn't happen much; it can be several months or more than a year between incidents. Every time, it's the same outlets. This morning, I went to unplug the wi-fi router from a dead outlet so I could plug it into a live outlet, and then all the dead outlets came back on, so is it that outlet that's causing all the problems? I'm planning to get an electrician to look at it, but it would be nice to have some ideas as to what could be causing it. I don't want him telling me the whole house has to be rewired if it's just one bad outlet.