In case it's a bad connection between the breaker and the buss bar: If you have a reasonably good voltmeter that will read millivolts, check the voltage from the incoming wire to the buss bar. You need the panel to have a load to get a good reading. On opposite phases you'll get about 240 volts, but across a bad connection on the same phase, it might be a half volt or more, depending on load.
