Hello there, sorry to hear of the bother you're having!
This is usually down to a bit of crud on the O Ring sealing surface in the cylinder block - it can be very hard to clean up / see from all angles, and all it takes is a bit of detritus or pitting and you'll find you get a very annoying intermittent leak.
I'd inspect the mounting hole closely with a mirror to help you see all around it - if you have pitting it can be resolved by carefully refinishing the metal and then using a slightly larger O Ring (which we will of course supply free of charge).
Keep us posted with how you get on - we don't always see posts on here, but you can email us at
[email protected] if I can help anymore.