Hi, Ken here, and I'll be helping you today!
First off, when you say "circuits" (plural), the problem must be with something they all have in common, which would most likely be at the main breaker or in that area. There are two possibilities: A loose neutral or a loose hot wire, or some other type of intermittent connection on one of those. A problem with the main neutral would affect ALL circuits. Since only SOME of the circuits are affected, we can assume that the problem is with the hot lead on one side of your main.
First, check the connections at the wire terminals on both poles of your main breaker. If they are loose and you can tighten them, then you probably have fixed the problem. HOWEVER, please observe if the conductors are aluminum. In this case, remove the conductors, clean the ends with something abrasive so that they are shiny, and immediately apply an aluminum conductive paste such as Penetrox anti-oxidant paste. Then, re-insert the wires under the terminals and tighten. Aluminum is quite often the problem.
If these suggestions above fail to fix the problem, then you likely have a bad main breaker. With time, circuit breakers
Working on the main side of the panel can be extremely dangerous if you are not familiar with electrical work. Try to find a way to temporarily disconnect the power ahead of the main, or have this work performed by a qualified electrician or electrical contractor.
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