Main breaker trips without others tripping

I am not an electrician, so please bear with me. For the last several nights (and only at night) the power in my house has gone out several times. When I check the main breaker panel, the main breaker (or maybe it's 2 breakers controlled with one "bar") has tripped but none of the smaller breakers (I guess they're subbreakers) has tripped. The problem has recurred several times a night, when only my wife and I are home and no large appliances, etc are running (and during the day, we use the microwave, vacuums, etc without problems). Our electrician got a new main breaker to replace the existing one, but this didn't solve the problem. We have lived here since 2002 (house built in 1997) and this has never happened before. We recently put in a security system, but unplugged the whole system (ADT) and the problem still occurred. We also recently switched from Verizon phone service to Cox digital phone service. The electrical company (So Cal Edison) says the problem is not between their transformer and our house, and said that for the past 24 hrs they got a low reading of 240 and high of 243. I apologize for this long description, but my electrician is "stumped" and I am extremely frustrated. I would sincerely appreciate any ideas about how to approach (and hopefully localize) this problem.