my Ford Transit van 55 plate , jumps out of gear !! ??? ?

If you look at the top of the g/box where the selector shaft enters the casing,if there is a brown/rust residue there then try oil/spray grease onto the shaft and work the shaft up and down and get some lubricant down into the needle roller bearings in there which rust and seize up and can restrict the movement of the selectors themselves,if this problem is ok then it looks like your synchro hubs in the g/box are getting worn.

cdc
April 2015