How can I fix Sprintshift on Sprinter 313CDi?

My sprintshift allways chages from "Auto" to 6th gear when driving at about 50mph, it sometimes changes from "Auto" to 4th or 5th when driving through the gears in auto. It seems to be OK and fully automatic if kept below 50mph. It has once got stuck with just "E" (I assume Error) and was a bugger to drive to local Merc Commercial Garage. It had a new Sprintshift pump last year. No messages on "plug in scanner" no one seems to know what the problem is. The Van is 2001 LWB 313cdi. Otherwise great van.