Why are the lights in my 60 year old house flickering?
Our house is old..1846 old. We had this issue in our home as well. All breakers were good, nothing was shorting out, tripping breakers, etc. 
First thing I did was call the power company. Turns out the service line connecting to our house was loose. They tightened it up free of charge and the problem stopped.
This can happen over time, after a storm with lots of wind, from a branch falling and catching it some, etc.
I'd have the power company check the service line and if it's all fine and good, then I'd call and electrician.