Stalls on acceleration or going uphill?

The battery light started flickering while driving under load, followed by stalling. The check engine, battery, and oil lights all came on. Able to restart after a few moments. My friends OBD scanner came up with these codes:
P2096 Post Catalyst fuel trim, System too lean Bank 1
Short term fuel trim -2.3 to 0
Long term fuel trim -3.1
... Which my friend thought pointed to the oxygen sensor misreading the fuel/air mixture and starving the engine. Our mechanic diagnosed a bad alternator. We replaced the alternator and the oxygen sensor. The stalling issue persists, although no more dashboard lights, before or after stalling.

The Elantra idles smoothly and revs without problems, which only occur on the road. New plugs were installed a month ago, and no misses. Also replaced a leaking valve cover gasket and changed the oil.

The car can creep along, but is not safely drivable, so I can't even get it to a shop without a tow. Pending your response, our next step is to tow it to Hyundai, as there are a few outstanding recalls that they have to do.

We don't want to keep replacing parts without a more definitive diagnosis. Money is also a concern. It would be great if you could identify the cause of the problem, and how to fix it.