If you have already done this I apologize.
1.Not fed on a multi wire circuit is it?
2.I would swap out with another arc fault breaker.
3.I would kill the circuit,see what is dead,go to the halfway point of the circuit and open it up ,seperate the wires,energize it see if it holds and reapeat and continue to narrow the possible location down.Sounds like there is a neutral problem or an actual arc problem.
4. older home is there lighting on the circuit?could they be switching a neutral for light control? (common in K&T wiring)
(don't know if that would cause the arc fault to trip though.)
[This message has been edited by andyp95 (edited 09-27-2004).]

[This message has been edited by andyp95 (edited 09-27-2004).]