AFAIK you can use the same shifter (Alfine/Nexus), it's usually the body that attached to the hubs that need to be right, as I've recently trying to find the right one for a customer's bike due to several variations (including a Scandinavian version).
What happen with the Jtek? need a bit more information than "just slipping".
Is it the cable or is it the lever that not staying put?