Fix my truck

[samd95gasser]

I just bought a 1995 dodge ram 2500 with s 5.9l magnum. It had a blown trans which I rebuilt. I got it running and driving and new exhaust. After it runs for awhile it likes to act up. It will drop in rpm's and eventually die if you nt constantly giving it gas. I've read the codes on it and it only says it's an o2 sensor. I've unplugged it and still no change in behavior. Another problem it has is it like to smoke in the through the vents. I would like to fix these problems so I can get it back on the road.

Please and Thanks