Loss of power at breaker
Ironically your wiring system has reached 30 years which is its half life. Meaning from here through the next 30 years an average system can experience 50% failure of it's components. This is just a statistical average and does not apply to any one system. 

I would start by performing some maintenance on the panel. First I would remove every conductor from it's panel connection, including neutrals and inspect and clean the conductor. Then with a voltmeter carefully turn it on and check to see if you are getting 120V on each pole of the breaker to the ground bus. Turn the breaker off and reconnect the circuit, turn it back on and recheck the circuit to make sure your voltage is OK. Then repeat all the way through the panel. I would start with the main breaker and go all the way through.

Second as long as I already had the panel open I would remove each breaker and check the resistance through the breaker with an ohmmeter. the resistance on all breakers should be consistent throughout the panel. If I experienced a higher resistance than normal or an open circuit, I would replace that breaker and I would check the connection to the bus for any corrosion, coloration or arcing. I would also do the same at the breaker conductor connection.

Then once I have reassembled the panel. Perform a voltage check by measuring the voltage from the main service feeder to the ground and the neutral, and check it against the voltage between the breaker post and the neutral/ground to make sure it is consistent with the incoming service voltage. It should not vary much more than one volt.

In short you are indicating that the panel connections are loose and it is not just with one circuit. So lets clean it up and see if we can get it back to proper working order before calling in someone for a complete panel replacement. 

Good luck and stay safe.