st33ly4

my engine management light is on with code 41 (primary heated oxygen sensor) i have had it replaced too times now light is still not going off. Got them off EBAY, that mite tell you something lol. Got a crack in the manifold aswell but got a new one on the way. Car starts okay. idles at 1000/1400 but once its heated up and i give it a wee rev it jumps up to 1500 or gos up itself and just stays there, if a rev it too say 3000/4000 it gos down to 800/900 then the revs jump back upto 1500+ revs. can anyone help me with this problem