2001 Hyundai Santa Fe Catalytic converter

When I purchased my used hyundai, it would shift funny and would slow down quickly when taking my foot off the gas, almost as if a break was stuck. The engine light was also on. After taking it back to the dealer 4 times for the engine light and many other things, they told me they found no problems but put new front brakes on. Four months after my purchase, the car would only go up to 20mph and the dealer told me one or two of the three cats were plugged and extended warranty would not cover the repair and neither would Hyundai because it was 700 over the covered 80,000 mi warranty. I took it to my regular mechanic and 1,300 dollars later I had my Hyundai back. 5 months later the transfer case needed to be replaced and a week after repair it starts losing rainbow colored oil underneath and the catalytic converter needed to be replaced again. My mechanic said it will probably get plugged again but doesn't know why. What is causing it to go bad?