OK please help been at MAG in Columbus Ohio with my X 5 with 143,000 miles on it

Symptons are if I start it up and the engine is cold when I put it in reverse or drive the rpm's drop below 500 rpms and it wants to stall

After it warms up to normal operation temp its fine and will not duplicate the stalling

It drives fine and I can drive it during this process

It was told it may be a torque converter causing my issue but if it gets warm in 10 minutes of driving and will not do it again could it be something else?

I just dropped $2,000 and not fixed yet

Could a bad thermostat cause this to happen as well?

Again when I put it in reverse out of the garage it shakes and then stalls I then start it and it starts fine.

Thanks in advance