My main breaker trips usually when the hot water heater is running or the dryer is on. We had this problem a year ago and we replaced the main breaker. We have not had any problems after it was replaced until 3 weeks ago. I am wondering if something is making the main breaker go bad or if it is something else all together?

You may want to have those specific circuits tested as well as a load test on how much power you are using, if you are using over the limits for your main, it will trip.

Too often there is a loose bolted connection in the busbar of a panel, and a breaker is changed and the problem comes back.

Heat can migrate from the bus into the breaker and cause a nusinance trip.

Having the load checked to be sure your not exceeding the rating of the breaker is the simplest, and wisest to do.

Just wanted to offer another possible reason for the tripping.

I appreciate the advice and will get those things checked.

In addition to the very good advice from TK and FR I would assk just how large is your service now? (amperage, that is)