Troubleshooting Circuit Breaker?

I have problem; my main 32 amp two pole breaker 220v is sometimes tripping and sometimes working without any problem. Mostly it's tripping in the early morning when I use geyser (water heater), but the breaker is not tripping when I switch on, it's tripping after some time. So I had an electrician visit and check for loose connections, but there are no loose connections at all. I replaced with a new geyser, but my problem is still the same. How can I sort out the exact problem.