merc sprintshift gearbox?

hi i am having a problem with my gearbox,the first sign of any problem was when i stoped to fill up,on restarting the van it started to vibrate and shudder on tick over,and it would not engauge in gear.i was towed into m/b garage and they said it was either the injector or it could be the flywheel.any way after 2 days they said it was the flywheel.at a total cost of 3000 euros.this work was done,but now when i kick down the gears work fine but it will not drop back up to top gear,you have to speed up to 82 mph.and then it drops back to top gear,ie 6th gear,m/b have had the gear box out again and told me they carnt find any problem,again at a great cost.any help please.