Subaru Repair: recurring check engine light on, gasket leaks, catalytic convertors

Question
I have a 2000 Subaru Legacy GT Limited Sedan with automatic transmission and88,000 miles. The Check Engine light came on last November. I took it to my local mechanic who diagnosed it as malfunctioning catalytic convertors, and both upstream and downstream O2 sensors. I then took it to a Subaru dealer and they diagnosed the same problems. I went back to my local mechanic and he replaced both sensors and the two convertors with aftermarket parts. After 1000 miles, the light came back on...the diagnosis was a "heater" malfunction in the downstream sensor. The mechanic removed that sensor and replaced it with a Subaru Factory Authorized Part sensor. After 500 miles, the light now comes off and on intermittantly while driving. It may stay on 3 to 4 days and then go off for three to four days. The diagnosis is the downstream O2 sensor is..."slow reading". What is going on?....what does "slow reading" mean?....could there be some sort of minor gasket leaks around the new convertors that causes the downstream sensor to think there is a problem?.....do the two new sensors and the new convertors have to somehow be balanced with each other so that the computer knows everything is ok? Any advice would be appreciated.

Answer
The repairs should have been made with Subaru parts as they are in some cases "updated parts".  I would never suggest the use of an aftermarket catalytic convertor at all.

What to do now?  It needs a real diagnoses and not a code scan, there is a big difference. A test with a lab scope will identify whether or not the o2 sensor is faulty.