T.fleming01

I have a 1999 xj sport that recently has had some problems. First off, my jeep has had a check engine light on for a downstream o2 sensor ever since it was handed down to me by my dad. I was driving home from school the other day when the check engine light went off and my oil pressure gauge dropped to 10 psi. Immediately I pulled over and shut the jeep off and let it cool down. My dad had the problem before with his oil gauge fluctuating, but never dropped and didn't move. As I was driving, the check engine light would flash and the jeep would fall on its face and fall from 55 to 25-30mph pretty quick. I wasn't far from home so I drove it home after giving dad a call. I got home and searched forums and found nothing about the jeep "falling on its face" and I searched for awhile. I went ahead and bought new up and downstream o2 sensors, an oil pressure sending unit, a oil pressure sending unit harness plug, and tps (I already knew it needed a tps because of the way it idled) once i got all the parts on, plus an iac valve, it started reading 0 psi and then the check gauges light came on. We tried wiggling the wires that go to the sending unit and nothing happened. So my question is, has anyone had this happen? My jeep is my daily and it needs fixed. I am going to put a mechanical oil pressure gauge on it (that goes in place of the sending unit) to make sure it has pressure, even though I'm pretty sure it does. I'd rather be safe than sorry though. Any advice helps! Thanks to anyone who tries helping!