Crunching gears randomly

[toenheel]

Morning all

Cecil every now and then won't engage a gear and crunches it, I've had it to bmw but it's intermittent so can't get it to do it in front of a tech. I've replaced the salmon relay. I've got full warranty on it but they can't claim until something fails apparently. Any ideas? Makes you look a right wally when you boot it past someone and its crunching and screaming up the revs!!