I replaced an old 109 amp panel, with a new 100 amp main breaker EATON with 20 spaces. Now my wife says the electric goes off and comes on. Can anyone think what could be the problem?
A loose connection could explain the problem, and could also cause a fire. You should have the installation inspected by a professional.
First, go straight to the panel and double-check your neutral connections into the panel. While not the most likely explanation, it is an immediate threat because a loose connection here can put 240v on appliances built for 120v, frying them and potentially starting a fire outside the panel.
Second, you are on the right track re-examining your work and asking questions and learning more. Learning is fun, and there's always more to learn, and I'd hit up the library and read whichever electrical wiring book speaks to you. The problem is certainly in the panel and your work, since that is what changed, that is the place to look.
If this same problem existed before you changed the panel, and changing the panel did nothing, that is a different kettle of fish. That means the problem is either with the power company, or with individual circuits. I would turn off every single circuit you don't absolutely require, and see if the problem goes away. Then turn on half the "off" circuits and see if the problem comes back. Then turn those off and turn on the others. If one of those groups fails, turn off half that group, look for symptoms, then thrn off the other half, etc., keep narrowing it down til you find which circuits are acting up, then follow the circuit.