Hi Lyn Jones
Having had this problem with a 3 speed box on my 1914 3 1/2, can I suggest that you check that the rod to the gear box is adjusted so that it is positively getting into the low gear. I had to bush all the link points to ensure maximum travel in the change lever. This cured the box jumping in and out of low gear. I also had a similar issue in top gear, but that turned out to be the dogs being worn on the two gears, they needed undercutting to lock together properly.
Hope this helps.
Cheers, Graeme