Jammed in gear

I have 52 w210 240 which has recently occasionally started to lock itself into gear after overrunning down hill. You feel a hefty thump as the box changes down and afterwards the gear is locked in and even using manual shift does nothing. The only way to unlock the box is to switch engine off and restart which clears it.
Less than 500 miles ago I changed gearbox oil and filter hoping this would sort it
Local garage suggest new box of electronics in box at cost of £1500-ouch!
Any ideas please