[habbyguy]

If it's designed for "cleaning catalytic converters", give it a try. Can't hurt (though the odds are not overwhelming that it'll fix the problem).

There are a few things that can cause this kind of problem...
1) A bad (worn-out) post-cat O2 sensor. I tend to suspect the sensors when they are well over 100,000 miles, and they're relatively cheap and easy to change.
2) "Gunk" on the converter's internals due to oil burning and/or misfires. Both can overwhelm the cat's ability to "burn itself clean". This is where a cat cleaning product CAN help, as can a "spirited drive" with a few long, hard pulls (which should dramatically increase the temperature of the cat, giving it a chance to clear itself of the crust on the internal grids).
3) A worn-out catalytic converter. This is the most painful option of course - lots of thread on it so I won't pontificate here.