tenacious check engine lite.HELP!

[MC007]

Hi All, I really need some help here. I initially got the check engine lite for Catalytic convertor efficiency problem when I was driving on the hwy for an hour. So I replaced the stock Cat with a DEC replacement unit with the old O2 sensors. THe problem came back again after another Hwy drive. So I got a new DEC replacement unit and this time I also replace both O2 sensors with new ones. It drives fine without the Check engine lite for 3000 miles and today the problem came back again.
This really bothers me, I have changed cat, O2 sensors and even bought another used stock muffler at lower milage when I replace the cat. My question is, what could possibly go wrong here since I have replaced all units that could be at fault.

thx

MC