As long as all your o-rings are intact, you have no leaks or poor fixings or threads, then I would suggest that you change components one at a time until you find the problem, and suggest before going ANY further you change the QEV. You will have trouble putting back on the original banjo as the line needs to be cut by about 2mm in order to fit the QEV. Someone on here should have a QEV to replace yours. Put a wanted ad up stating what you want.Just that Marcus Geezer on here had a very similar sort of problem on his ion many moons ago, we tried everything, batteries, resetting the dwell, cleaning, lubing the marker and reg, in the end it turned out to be a dodgy qev, once this was replaced and the marker set back up again it was fine.
see if you can lay your hands on a banjo fitting to test it with before you shell out on a new QEV
Be methodical about your approach, and after you have fixed the problem, which you WILL do, you'll wonder what all the fuss is about and will have learnt a load about how your ion works to boot.
EDIT: Have you changed all the o-rings. They can sometimes swell if old causing this problem. It's happened to me!