another O2 sensor problem

Well the story begins like this. my cel came on for bank 2 sensor 1 slow response. so i replaced both front sensors thinking it would be nice just to have new in the front. i reset the ecu and 150 miles later bank 2 sensor 1 comes back on slow response. wtf is brand new. so im like alright maybe i got a bad sensor so i swabbed the front ones. and it came back on bank 1 sensor 1. so i assumed that i bought that one bad. before i could replace it the cel light went out. 300 miles later its back on. now it says both bank 1 and bank2 sensor 1's are slow response. wth is going on? what does slow response mean does any1 have a clue how to help me?