Joy,

I had a very similar problem when I first installed my new engine last spring.  I drove with my O2 sensor unplugged for a trip to Santa Fe and on the way back I got the rich, low mileage, stalling problem you've described.  I reconnected my O2 sensor and that problem went away.  I don't know why it took it solong to surface.  Hope your problem is this simple.

There are three connectors for my O2 sensor; one is flat and two are round.   Just hook them up and see what happens.  If after driving for a while nothing improves, get out and switch them around.

Good luck.

Warren Lail
88 Westy "Billy Bones"