I have been using South African .223.  Starting with a clean weapon, I start having double feeds (failure to extract/eject and second round jammed underneath) after about 120-200 rounds.  Last session, same results.  Conducted a field expedient cleaning (WD-40 and cleaning brush) to remove a build up of residue around the gas tube.  Worked fine after that.  Is SA ammo "dirty" or is it SOP to have to clean after 200 rounds or so?  BTW: this weapon is a Model-1 Sales 20" ES2 kit modified into a 16" dissipator on a Colt lower (the Smith opened up the gas port under the front sight to compensate for any gas loss due to barrel shortening).          

I've fired up to 750 rounds of SA in one day through the same weapon w/o cleaning and never had that problem.  Your gas port might be a bit too small.