Hi Insane. How many miles are on your trans ?. Have they been "hard" miles ?. If your trans is high mileage or getting tired, a shift kit won't inprove it much. Did you put in a new batch of trans fluid when you installed the kit?. When you pulled the trans pan,was there any debris in the bottom?. If so,it could need a rebuild. With an O.K "high mileage "trans,try some "Ford" grade trans fluid for C4-C6,it's a bit thicker and will increase line pressure a bit,GO CAREFUL !!!.
The Street shift kit should give you a firm to hard 1-2 change and a firm 2-3 change.This IS NOT taking into account Engine HP and Stall Speed which will affect shifts. The competition version is "Hard" on ALL shifts and not pleasant for everyday driving.
As far as not wanting to stay in gear,I think this would be an adjustment problem with the cable kick down.
Hope this helps........Cheers.....Barry.M