RE: Welcome to Club SAITO !
my first thought would be a hotter plug, like an OS-F... but you may have already tried that.
That could explain how it would run nicely at idle with the cylinder pointing up, but drown out when the cylinder points. down. You might also look at your tank pressure line.. make sure that it's connected well and providing good pressure to the tank. Other than that, a trip back to Horizon might be in order.