Was the house 15+ years old with an ITE/Gould panel? If so check the buss connection with the breaker and I'll bet some are burnt. It doesn't have to be ITE, but that's what I've seen most. I've also seen the same symptoms with a bad main breaker and a bad meter base.
Do you have an infrared thermometer? If so this can easily find loose connections because they will be much hotter than others.
