94 z28 A4

Ok.. I have a 95 z28.. a couple weeks ago it started running rough and would black smoke if I got on it hard. I had it scanned and it was passenger side o2 circuit. I changed the o2 sensor out with a brand new one and it ran perfect for about 2 weeks. Then one night it started doing it again. So I pulled the pass. side plugs and they were all fouled out. Changed all 8 plugs and now it will run good until I get about a mile down the road and the car is warm then it starts running really rough like it's not on all 8 cyl. No black smoke though. I keep driving and a little ways down the road the check engine light will come on and it clears up and runs great. Once I shut it off it does the same thing until the check engine light comes back on and then it runs perfect again. Any help would be greatly appreciated!