I was driving a POS rental Ford Taurus that had the same symptoms. In one day of driving the damn thing just got worse!
At first it would shifft kinda hard into first then I noticed that when I floored it, the trans slipped as the revs rose in 1st gear then as it was about to self destruct, second would engage really hard that the front tires would chirp.
Then it just got worse and all the gears started doing that.
Never really knew what was wrong but I knew that the transmission started leaking some transmission fluid.
..just to make sure, have you checked the transmission fluid level with the transmission and engine up to operationg temp (and engine on) w/ the trans in P?
2008 BMW 335i Coupe