ok i replaced the O2 about 1 week ago and i still get the code 13 when i am cruising on the freeway and i have reset the ecu about half a dozen times since then. it only happens when i am cruising though, stop and go no problem and i have to be cruising for a while before it shows up (bout 5 min. minimum) then when i slow down or speed up the SES light goes out but of course i still have the code.

any thoughts on why this is happening.

 

The wire itself could be shorting out somewhere. Also, it is entirely possible that it is an intermittent break in the wire/sensor itself. I had a new O2 out of the box that would intermittently stop sending a signal (verified w/ a DVM while driving.)
First step is look over the wiring harness and make sure everything looks good, nothing cut/burnt/chewed on by mice. Then i'd jumper into the O2 wire at the connector and run the wire into the car and watch the O2 voltage w/ a DVM. That will tell you the whole story, if the O2 voltage looks fine/normal and doesn't cut out or anything then it's either ahidden wire problem, loose ECM connection, or i suppose even a bad A/D converter in the ECM.
...ed