My son has a 1999 Chevy Malibu with a 2.4 liter engine. He said the car had stalled out on him a few times in the last couple days but it would start back up for him. I drove it to work today and it seemed to run fine on the way but I did think it stumbled once but only for a second or two but it did not stall out. The drive home was a totally different story as the car just quit on me while driving on the interstate. It seemed to hestitate then lose power and then the rpm'* dropped to zero and some dash lights came on. I was able to pull over and it started right back up and I went on my way. Shortly after that that transmission seemed to start acting up as it was down shifting late and was shifting hard sometimes. The car did not seem to be running right even before the tranny started acting up and it then stalled out again while driving and this time it didn't start up right away. It took several attempts to get it started and keep it started but I was able to drive the rest of the way home but the tranny was shifting late and hard. I scanned for codes when I got home and pulled codes P0122, P0171 AND P0335. I need help bad.

Well lets start with the codes...

P0122 Throttle position sensor performance
P0171 Fuel trim system lean
P0335 Crankshaft position sensor circuit

I wouldn't worry about the P0171 yet it'* more than likely a vacuum leak caused by some rotten vacuum hoses, but unless it'* a huge leak it wouldn't cause the car to cut out on the highway, I'd get the other two figured out first. The throttle position sensor will cause the transmission to increase line pressure as this is one of the main inputs to the transmission to know when to shift, downshift etc. If this sensor screws up it can cause the transmission to do some weird things! I have replaced these on these engines. Good idea to do a wiggle test and inspect the terminals for corrosion or ing. Wouldn't be the first TPS I've ever seen changed on one of these though.

The crank sensor problem could be the crank sensor or the ignition module. Basically the ignition module is the middle man between the PCM and the crank sensor. If the ignition module intermittently does not process crank signal it will set a crank signal code when it'* actually the ignition module. Just replaced one on a Saturn due to this problem, but it was setting a cam sensor code, same idea. Still could be the crank sensor though or on even more rare occasions the PCM. Again with the engine running wiggle the wiring around under the hood near these components to see if it stumbles and check your connections before attempting to repair any components.

May be a good idea to take it to a garage to get it properly diagnosed if you don't have any diagnostic equipment. It may be a lot quicker and cheaper than throwing parts at it.

Sounds like the crank shaft position sensor to me.

Could that cause the tranny to act the way it is as well?

No it would not. You may have a seperate issue related to the throttle position sensor.

Here is a test procedure for the CPS:
Part 1 -GM 3.8L Ignition Module Test and Crank Sensor Test (3X, 18X)

Here is a test procedure for the TPS:
Part 1 -Throttle Position Sensor (TPS) Test: GM 3.8L (P0121, P0122, P0123).

Really sounds like the TPS could be the problem as far as the tranny acting up. Could this also cause the car to stall out like it did?

It should affect performance, but I don't think it will stall the car out. Read the links I posted.

Sounds like that might be a good place to start. What part if any could the ignition module play in all this as I see it was referenced earlier in the post and I have a spare one on my shelf.

I don't think the ignition module is the issue, but if you have a spare it wouldn't hurt to pop it on. If you didn't have one kicking around it would be a different story.

I replaced the ignition module and coils since I had those spares. Took car for a short ride with no problem but again it wasn't a very long drive. Scanned for codes when I got back and only got the P0171 code. P0122 and P0335 are gone. Any thoughts on that.