Sprinter 315 2008 manual 6 speed gearbox problem?

Hi sprinter jumps out of first gear , tried changing oil to atf which did help from cold start but seems to get worse once the box heats up. Does not do it every time and is ok if you hold it in on gear change. Checked cables seem OK with no wear. Low mileage van, less than 100000. Can it be the counter weight that sits above the box , linked to the horse shoe assembly, this seems very loose and flops from side to side when I disconnect the small linkage. It seems to me that first gear just needs a tiny bit more holding power to solve the prob.!
All gears getable with engine off though first can be stiff.