The tarns works good at low speed and shifts fine but I noticed at highway speed if I step on it the trans slips. Can I service and get more miles or is it done???

Sounds like it is on the way out but try a fluid change (DO NOT FLUSH IT AT THIS POINT!!!), if it helps do another change as you only get about 1/2 out when you pull the plug, then try this product. Seafoam Trans additive. If it is salvagable this will do it and won't cost ya more than $50 or so to do it all provided you do the work yourself.

I changed the fluid and then drove it a bit. I paid close attention to what was going on and tried different speeds etc.

 

When on the hwy in 4th gear, if I push the peddle down firmly it will shift to third and slip (maybe not shift all the way). If I slowly push the peddle, just enough for it to shift or if I shift it manually it will shift to third and won't slip, it feels good. Does this sound like a problem with the duty selenoid? Would this be something gummed up inside the trans or a poss electrical or computer problem?