What is causing catalytic converters to clog?

2004 Ford Escape XLT, 3.0L V6, 98,000 miles.

4,000 miles ago, I had a new rear catalytic converter put on my Escape b/c it had become clogged.

I recently replaced all six plugs and coils b/c a few cylinders were misfiring. After doing that, I was soon was getting engine codes from two O2 sensors for 'lean' gas. So I checked for vacuum leaks, verified all hoses and connections were ok, the manifold bolts were torqued correctly, and I could not find any leaks. I replaced the PCV valve (the old one barely rattled anymore)so I am hoping I fixed the 'lean' engine code issue.

Now, the same converter is clogged again after only 4,000 miles. My questions.

Did the converter possibly become clogged b/c of the cylinder misfires and 'lean' gas issue?
Are there other underlying issues I could check out that may be causing the converters to fail?
I am not sure if I have solved the problem with new plugs, coils and a PCV valve, or if there is something else going on that will destroy another converter.

Thanks to anyone in advance for your expert advice.