has anyone ever had this problem?

My 93 1.6 sentra sometimes runs very rich, and the car hesitates badly when it happens. Sometimes the car runs great, and sometimes it runs like the O2 sensor is bad. I replaced the o2 sensor twice and no difference. If it was the o2, wouldn't it always run poorly? The car when running bad has a bad smell coming from the exhaust, so I thought it might be the cat. but it isn't.I had it checked. Any help would be appreciated. Thanks

Have you checked the throttle position sensor? What octane level do you use?

Also, have you hooked your car up to a computer to find any error codes?

I use 87 octane. No to throttle postion sensor and computer.I'll see if someone can do that tomorrow thanks.

Did you find anything?