I have an SHDxxx G21 Gen4, test fired on 11/16/2011 which only acted funny if I loaded ammo on the minimum range. At 850 fps with 230 gr. projectiles, and 950 fps with 185 gr. projectiles, it works like a champ.
It's been my .45ACP test platform for the different powders and bullets I've tried. Additionally, I haven't had any problems with factory ammo at all.
I hope that you can get yours working well, because this model is also my favorite of all the Glocks I have.