stalling 3800 in 92 Pontiac Transport

I have an annoying intermitent stalling problem with my wife's van. It seems to stall mainly when it's cold and after 20 minutes of warmup does not stall again. It will stall when at idle or when driving down the road. When it does stall while driving it doesn't sputter at all it just dies immediately. After a stall it does take a lot of cranking to get it started and when it does start you get a puff of black smoke from the exhaust. The engine light come on intermitently also. After restarting from a stall quite often the idle will be high (1500 to

3000 rpm), the tranny will not drop down into low but will shift upwards normally and the speedometer will not work. After a few minutes of this it will always correct itself. The crankshaft position sensor was replaced about 6 months and should be good. I took it to a garage and they checked the computer for error codes and it listed some codes that had something to do with the torque converter, but they weren't sure if this was associated with my stalling problem. The garage could find nothing that would cause the stalling problem that I have. Any ideas at what I could have checked next?

seems to stall mainly when it's cold and after 20 minutes of warmup does not stall again. It will stall when at idle or when driving down the road. When it does stall while driving it doesn't sputter at all it just dies immediately. After a stall it does take a lot of cranking to get it started and when it does start you get a puff of black smoke from the exhaust. The engine light come on intermitently also. After restarting from a stall quite often the idle will be high (1500 to 3000 rpm), the tranny will not drop down into low but will shift upwards normally and the speedometer will not work. After a few minutes of this it will always correct itself. The crankshaft position sensor was replaced about 6 months and should be good. I took it to a garage and they checked the computer for error codes and it listed some codes that had something to do with the torque converter, but they weren't sure if this was associated with my stalling problem. The garage could find nothing that would cause the stalling problem that I have. Any ideas at what I could have checked next?