Ken;

If you had a CAT fail,there was probably a pre-existing problem in the vehicle that may be more apparent now. If you had a clogged up CAT previously, the usual result would be a general severe lack of power; probably the vehicle would not go over a certain speed without difficulty due to the exhaust flow restriction. Now that the flow restriction has been eliminated with the installation of the new CAT you may be able to accelerate more rapidle and reach a more normal driving speed, making other problems more apparent.

Cats don't fail for no reason; they should operate indefinitely unless some external cause induces failure. They have no moving parts to wear out. The most common causes of CAT failure are an engine misfire condition or a rich fuel mixture that goes uncorrected for a long period of time. What happens is that nay unburned fuel from either problem travels down the exhaust pipe and is oxidized (burns) in the converter. This rapidly raises the internal temperature, sometimes approaching 2,000 degrees F (that's why all the heat shields around the CAT: to prevent the floor or nearby grass from catching fire). When internal temperatures reach this level, it exceeds the melting point of the ceramic honeycomb substrate inside the converter shell, causing it to melt and break apart into chunks from the vibration. These pieces then tend to roll to the back, obstructing exhaust flow and usually making a noticeable rattling noise.

The shop that diagnosed and replaced teh failed CAT should have also diagnosed and repaired the cause of the failure as part of the job. If they did not do this, it is very likely that your new CAT will fail also, because the same engine problem still exists that caused the first one to fail. What you are noticing now is probably symptoms of whatever problem caused the first CAT failure, perhaps getting worse.

There are many possibilities as to the cause of the engine performance problems you are experiencing, and diagnostic testing will be required to identify what system the problem is in, and then narrow it down to the root cause. It could be something as simple as worn out spark plugs and wires, or more esoteric such as poor cumputer ground connection or a failing engine controller.

I would recommend taking the vehicle in to a different shop; one that is experienced and competent at diagnosing performance problems. In your area, this may mean your GM dealer service dept, although there are many good independent shops with the proper training and diagnostic equipment out there. It sounds as if your present shop is not doing a very thorough job for you. Just installing the high profit big bucks easy to replace part without investing the time to find and correct the root cause of the failure is not a very professional approach, and not doing you a good service.

I hope this is helpful; if so, an accept would be most appreciated! thanks!