2005,S80,2.5T, I suspect a transmission problem

[griffin90]

I have a 2005 S80 2.5T with 168K miles. I am trying to get some idea of what is wrong before I take it to mechanic. When first started the RPMs go to about 800-700 and engine stutters and acts like it is going to stall. It also stutters and stalls when first accelerating. It seems to get better as engine warms up. I don't know if this next symptom is part of the same problem or something completely different. When coming to a stop there is a loud bump and vibration that feels like it is in the front wheels, transmission. It's been doing this for two to three months and is getting worse but hasn't shut us down yet. I am getting messages "Time for regular service" and "Engine System Service Required". I am suspecting transmission problems, possibly torque converter, or maybe throttle body causing the stalling when started. After reading this forum I am not sure I can trust shops and I would like to hear suggestions so that I won't go to the shop unprepared.