Lights flickering every 15-20 mins.?

I suggest you ask neighbors near you to see if they are noticing the same thing, this would tell you if it is just you or the utility side. If none of the neighbors are having this problem I would call the utility company and ask them to send someone out and check your connections and see if it is on their side some how before you call and electrician out and have your home checked.

This could be anything from a loose connection on either hot wire or a neutral wire in your panel to your meter base to your connection at the utility as well.

There is a lot of talk these days about Harmonics in residential loads because of the unbalance of so many neutral loads in many areas and if this is the case (rare) they can adjust this with a neutral indicator on the utility side of the transformer or even replace it if needed.

Some homes do have some load blanking as it is called but not normal and not all the time.

If you notice the lights flickering when a motor ar compressor is kicking off and on it tells me that you have a loose connection on the neutral side some where usually, but not always.

In any regards have it check before it gets worse and sometimes it can be costly on electronics and other things if not repaired soon.

In addition to what Peter noted, also have the electrical company check your meter, itself. As the meter begins to fail, this exact condition occurs.

This happens frequently in my own neighborhood as most of the homes are reaching 30-40 years old and the meters are failing.

I'm no expert, but my friends mother had that problem or else a similar problem and it turned out to be mice in the walls. Not saying you have any, just trying to give ideas as to what the problem might be.