The dip stick for the tranny goes into the tranny pan. Being overfull should cause some leaking (blowing out the vent) and could likely cause slipping. You need to correct the fluid level. I use a small hand pump and just draw the excess fluid out the fill tube (where the dip stick goes) until the level is correct.
Try to lower the level to 10mm below the low line when cold then see how it does. If still slipping try the fluid & filter change.
'73 280SEL 4.5 (9/72)- RIP
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2012 VW Jetta Sportwagon TDI...at least its a diesel
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