What would cause some lights to go off that are on the same breaker? Breaker did not trip.?

Breakers are 2 100 amps that run my furnace, basement lights and elec. hot water heater. Lights were going off and on, furnace and hot water heater stayed off. Is this possibly faulty breakers or a more insidious problem? Would not surprise me, this house is turning into a money pit! I turned the breaker off, can replace the breakers myself, but if its more serious than that I will call an electrician.