my girl friends 92 Lumina was doing that and I replaced the (aic) air intake control this fixed it.
it would run fine most of the time but like you said it would die when wormed some times and would start right back up.
it should be on top of the engine couple of screws not to bad to change.
when you read the code after it flashed 12 three times did you leave it hooked up to see if any more codes came up?
first time I ever did that I seen 12 three times so I unhooked it thinking their were no codes.
then a friend of mine did it and it read 12 three times then the other codes came up three times each before it went back to code 12 and started over.
seams I unhooked it to soon.