I think the transmission or differential is slipping, but not sure which. When stepping on the gas, occasionally, the car either stalls or is sluggish going into gear. What is the mose probable cause?
Well, it’s not the differential. This driveability problem could be stemming from the engine, or it could be transmission-related.
Can you clarify “sluggish going into gear”? Do you mean that you place the shift lever in the “D” position, but that the transmission doesn’t engage for several seconds, or do you mean that when you already in gear and you press the gas pedal, that there is a lag before the vehicle accelerates?
It is the latter. Once in gear, sometimes it hesitates to go forward after stepping on the gas.
Is the car up to date with all of the scheduled maintenance in the mfr’s maintenance schedule? That is always the first place to start if you have driveability problems, and that is what you seem to have, rather than a transmission problem.
If all of the maintenance is up to date, then I would suggest that you check the MAF sensor to see if it is dirty. Another possibility is the IAC, but I would think that the MAF is the more likely source of the problem.