1997 Geo Metro Engine cuts out when letting off the gas

I recently made two long distance trips and experienced some problems. On the first trip I was almost to my destination when my engine started stalling out when I let off the gas to shift. It did it repeatedly and even forced me to pull over to the side of the road to get it started and take off again. Each time it happened, I depressed the clutch, checked the tachometer and the engine was at 0 rpm. The dash lights also came on just like they do when you turn on the ignition but don't start the engine. A couple times it allowed me to start it again in neutral and then take off. I also waited a couple times (coasting) and it took off again on its own. Later that evening I drove all the way home with no problems at all but it was about 15 degrees cooler outside. The next trip I experienced no problems until I got stuck in a traffic jam on the freeway and it started acting up again. Repeated stalls and then I was forced to pull over and it would not start at all. After having the car towed back to my home the next day it ran and drove perfectly fine again.

I took it to Autozone and asked them to read the service codes from the computer.
The codes that showed are P0500 and P0171. On first glance the codes may indicate a bad O2 sensor and a problem with the VSS (Vehicle speed sensor)
HELP!
Thanks,
Jason