You think it's bad off the bench............
give prone position shooting a try using a sling. Smallbore competition shooters do all sorts of things such as using slings w/wide cuffs, padded shooting jackets, etc. to try to dampen pulse but probably most effective is to try to time the trigger break in between heartbeats.
Sounds as if you may be pulling rifle in pretty tight to be able to notice pulse from sandbags. Unless you are shooting a magnum with very heavy recoil, you might try letting the rifle rest on the bags and gently pull into your shoulder with your right (trigger) hand and see if this reduces the visible pulse induced movement.
Couple of other things you can do is cut caffein intake, especially on range days, also the better condition you are in, the slower the heart rate, giving more time in which to break trigger. Wish there was a short, easy answer, but..........
Didn't know it was a problem 'til ya put the 20X on but that really shows you what caused that last "flier", doesn't it. It will also show you the mirage and it's "benefits" when shooting HP long range w/scope @ 20X, or, for that matter smallbore @ 100 yds.
Regards,
hps