Most likely the cat is just getting marginal in it's ability to do it's job.
The cat monitor does not run all the time. There is a certain set of driving cinditions that must be met before it does it's comparison. Several other of the monitors must run first as well.
So sometimes when the cat monitor runs it passes and sometimes it does not.
It may help to understand how the cat monitor works and the relationship between the upstream and downstream oxygen sensors. A normally operating upstream oxygen sensor is rapidly and constanty changing voltage between 0 and .9 volts. The oxygen sensor generates voltage in the absense of oxygen. The less the oxygen, the higher the reading. 0 volts is a very lean reading and .9 is a very rich reading. This is the main information that the processor needs to determine the fuel quantity needed. The downstream oxygen sensor is used only to determine how efficient the cat is. Once the exhaust has passes through a properly working cat, the oxygen sensor readings will be much more consistant, that is they will not be rapidly switching from 0 to .9 volts. When the cat monitor runs, it compares the activity of the upstream sensor to the activity of the downstream sensor. The downstream sensor must move some, but if it too closely mirrors the activity of the upstream sensor the processor recognizes that the cat is dead.
What can you do short of replacing the cat? The following things might help, and they might not. Make sure that the engine is running effeciently so that there is not a lot of wasted fuel that the cat must constantly burn off. Marginal plugs or wires might make some difference. Make sure that the injectors, throttle body, intake manifold, valves, combustion chambers, and air filter are clean. Sometimes the cleaner that you run through the tank to clean the injectors or that you spray into or otherwise introduce into the engine to clean the injectors, intake manifold, combustion chambers, and throttle body also may help clean some contamination off the substrate of the inside of the cat and help restore it. Getting the cat up to proper operating temperature for a long drive also often helps clean off the substrate in the cat.
Severly overheating the cat can cause the substrate to break up and clog up the cat and sometimes the rest of the exhaust. This is not nearly as common as it was when cat systems were new. This would ususlly be caused by running extremely rich or with a severe misfire and keeping the engine under heavy load.
Lastly, some claim that they have overcome an intermittant cat effeciency code by replacing the upstream sensors. In theory, this may help if the sensor has become lazy and does not switch as rapidly as it did when it was new. The cat monitor would recognize the the downstream sensor was mirroring the action of the upstream sensor too closely.
Before jumping to too many conclusions, I would like to see some data logging of the upstream and downstream sensors. That may not be practical for you, so I hope that you may have a bit more information to help you determine a course of action.
