This was quite a typical YS problem some years back
I've had the problem 4 times on my old 91FZ.
Ring change cured mine every time. Even though the old one I took out looks perfect.
Was recomended to change the liner as they belived it was causing the probems. ie it wasn't perfectly round.
Nothing you could measure without some serious equipment... but enough to case problems once the ring beds in and wears some.
Strange but the info came from YS themselves.[sm=50_50.gif]

They started gass porting the rings some years back to help cure this.
Mind you, mines been good for some time now... still with the old liner and no porting.[sm=thumbs_up.gif]

Get a new ring and try it. If it doesn't work... least you'll have a spare ring.

And hey, you're in the UK!
Just send it to Phil at Pro Build if you're uncertain.
Give hime a ring.
You'll have it back in a flash.