I was doing 60mph on the motorway when my Hijet gearbox suddenly sort of lurched then the car totally lost power even when I changed down through the gears until I had to pull over on the hard shoulder. I tried starting off again and it was fine. Then it happened again - both times it happened when I eased off the throttle in 5th. I had been driving in traffic on a very hot day for more than three hours - dunno if that is significant. It seems the car is tending to slip out of 5th and 3rd - I can feel it juddering and have to hold the gearstick to stop it slipping out. Is this a synchromesh problem and if so is it going to be expensive to fix? If anyone has any ideas, I'd gratefully receive them. Thanks