Toyota Repair: Idle Problem, fuel filters, exhaust manifold

Question
I have a 1996 Camry LE 4 cylinder with 145,000 miles. Before the engine gets to running temperature the idle is fine. When driving for extended periods when I stop at a red light the idle is around 850rpm. The idle will fall to 750rpm or below and the car will begin to labor, sometimes stalling. There are times the car will stall when exiting the interstate and coming to a stop. I have changed plugs and wires, rotor button and cap, pcv valve and the air and fuel filters. Twice the check engine light has come on giving the PO240 error. Sometimes the idle increases when I turn on the air conditioner..sometimes it does not. This is not consistent and I can find no pattern to this issue. Thank's in advance for your response.
Richard

Answer
Are you sure it is a 240 code rather than a very common PO420?
The 420 refers to a catalityc converter problem, since this is an obvious problem I recommend taking care of this one first before replacing any other parts, the converter probably needs to be replaced.
The converter is part of the exhaust manifold manifold.