First, For the stalling, when was the last time it was tuned-up and filters changed?
Second, to check codes,
Remove the cover over the cig lighter
You will see the ALDL connector there. On the top left there should be two pins next to each other. Place a wire or paper clip and short out the pins.
Next turn the ignition to "run" (do not start the engine)
the CEL light on the das will start to flash codes.
The standard code is "12" so it will flash once, pause, then flash twice.
It will do each code 3 times and then will cycle back to the code "12"
Write down the code and let us know what they are.
Finally, the torque converter lock-up is a solenoid that goes bad. It is located under the side cover of the trans. The best thing to do is to buy a new trans filter and solenoid. Drain/remove the bottom pan and filter. then remove the side cover. The solenoid is easily changed. Just follow the 4-wire connector to the solenoid. Replace side cover and bottom cover. Fill with trans fluid and you are done. It can all be done without removing the trans and is just a matter of removing covers and a 10mm bolt for the solenoid.
NOTE: If you buy the lock-up solenoid from a GM dealership it will come with the connector and wires attached. If you buy one from an auto parts store you usually have to splice the wires of the solenoid to the connector. While the dealership solenoid is slightly more expensive, they seem to last longer than the aftermarket ones and wire splices on the aftermarket ones can have issues with trans fluid.
