Electrical surges, lights flicker/dim them go back to normal

I live in a mobile home, first issues I've had electrically. I noticed the lights would dim from time to time, a fan that was in the bedroom would run like it was on low even though I put it to the highest setting. First thing I did was called utility company to come check meter readings and such and he said they had good voltage even with everything on with no fluctuation. He told me to get a reading on the breakers in the box and see what I could come up with. Borrowed a good meter from work and checked the breakers, the main, a double pole which is a 100amp breaker (assuming since it says 100 on the handle for both) is the first I checked. The bottom one reads a constant 118 and the top would go from that range down to 78 during the dimming of lights. Is this just a bad main breaker that isn't letting the full juice through? Calling an electrician tomorrow but figured I'd get some feed back from here just in case someone else is having a similar issue. The home itself is only 15 years old. Possibly not balanced correctly in the box?