tried your ideas before, I think I know the problem and for 37 euros (by the way a Euro is a small Australian Kangaroo) I could let you in on the secret but as you probably could not afford 37 euros her it is. The EU seem to be liking Aussie things as the European Economic Community EMU is an Australian flight less bird called a Emu.
The GPS played up 2 years ago, same problem, so I asked myself if there was any factors that were the same and it was that I had ridden through heavy rain with my waterproof(?) GPS. I dried it out and guess what it seems to be working ok now. so do not let it get wet.