As ditchcrawler has said I had this problem on my nb with a Gardner 4LK engine and the 500 gearbox. It nearly took my teeth out when I changed gear, never mind the teeth on the gears. It was very audible to passers by on the towpath and was embarrassing. This was way back nearly 20 years ago. As an engineer I tried various home built solutions to try and soften the rate of oil pressure rise in the box including experimenting with a length of pressure hose to allow the enclosed air to be compressed as the oil pressure rose, then adding a pressure accumulator and pressure gauge (just to monitor pressures) but nothing really worked well. Then, having read about the control system that existed for soft slip purposes I thought that a similar system to ease the very sharp oil pressure increase could work and wrote to PRM about it suggesting my idea. I got a very helpful reply from the Technical Director who agreed that my idea was a good one but saying that a Dutch dealer had just developed just such a system with a twin accumulator and modified valve block to fix the problem and offering me that conversion kit to fit myself FOC. I fitted it and it completely solved the problem. It is a known problem but fitting the modified valve block with its two accumulators will fix it. In fact, at one time, I thought that I had heard that the boxes were only supplied with that mod already fitted but I may be wrong in that thought.
Roger
PS. I still have my unsuccessful accumulator/pressure gauge modded pipe in my garage now.