What would cause a brand-new EGR to blowout twice ?

It was running rough , kind of a hesitation every once in a while. Code suggested EGR so replaced it, then a month later check engine light came back on and it started running rough in addition to loss of power. The mechanic replaced the spark plugs along with several rubber lines which were showing age , and another new EGR. While returning my vehicle , the new EGR blew and a plastic sensor next to it melted. The mechanic thinks the catalytic converter must be clogged to create that much back pressure. The spark plugs were something we had previously discussed and agreed upon, and were not related to what is happening now.

Please see my other response ...... Thanks