I had something similar with my automatic 328i - started out as bogging, then eventually worsened so that under acceleration, the RPM's would over rev, then the diff. would catch, and 'chirp' the car forward, like shifting into Neutral, then slamming into Drive. Eventually, overdrive failed altogether, and I could only drive in 3rd
Turns out that the transmision was in 'limp' mode, and is now undergoing a total rebuild (!!)
I hpe for your sake that this isn't the same problem that you have.
On a related note -- anyone wanna buy a kidney?
silver 2006 Dodge Charger SXT - performance chip, blue filter Cold Air Intake, 18" chrome 10-spoke rims, custom graphics, leather, custom vinyl interior dress-up, chrome trim package, chrome grill, custom "Punisher" hood badge