First base is fluid and filter change, as low fluid will make it slip with sort of a surging motion (check fluid again just to make sure). Second step is probably the B2 piston - did you look up into the transmission to see anything awry? B2 band is right above the valve body and you can see it from the bottom. In your fluid and filter change, did you see any particles or anything out of the ordinary in the pan?

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth