Recently, we have experienced flickering lights at various spots throughout the house. As I'm typing this the lights just went off and back on and nothing else was on in the house. Our house is over 20 years old. Any idea where we should start investigating? It has never happened before.

I'd start with the light fixtures... try to see where the wires go into it and if any of them look like they are messed up, call an electrician. If you can't find it, it might be more serious and you'll need to call anyway. So my advice... call an electrician. You could electrocute yourself messing with stuff you don't know about.

This is a loose connection most likely at the panel, or can be in the utility connections outside. This needs an electrician, and possibly the utility company out to trace the problem.