Transmission...transaxle

[V6Vdub]

As the name suggests I have a turbo V6 in a VW, its basically half a Riviera with a VW as a hood...Here is my problems...The tranny will short shift. I mean I will run up to 6k then slam into second, the tranny shifts to second then immediately to third with the shifter still in second. The tranny is a TH325, and I have not been able to find anybody who can convert it into a manual/auto....anybody got any info....help. But wait there is more. Does anyone know if a GM posi differential of any type will work in that crazy tranaxle?
Thank you much
Kevin

 

[Ttype83]

Does it have a TCC?
Is there a downshift detent cable or a linkage?
early shifting can be caused by a bad TCC, TC, misajusted detent cable and/or linkage or the trans going south. If you have done a lot of 6 G slams, I would lean towards the going south thing