First check your gas cylinder screw, right underneath the muzzle. Is it nice and tight? If so, remove it, and thoroughly clean the threads. While you're at it, look into the gas cylinder. Is it clean?
If not, field strip the rifle. Make sure the op rod head is clean and serviceable--no big nicks, not corroded, no powder buildup. If there is baked on carbon, use a good solvent ONLY to clean it off. No abrasives, no scraping, no power tools.
Remove the gas cylinder assembly and lock by unscrewing the figure-8 that encircles the muzzle. Remove the gas cylinder assembly, and check out the gas port. There is a tool that is GI issue for ensuring that the gas port is clean--look into getting one of those, available from
CMP Home.
Reassemble the rifle--there should be NO lube inside the gas cylinder, and a very light coat of oil on the op rod itself--none on the gas piston.
Try test firing again. It sounds like the friction on the bottom of the bolt from the full clip is inducing short stroking. When the friction lessens--less rounds in the rifle--it functions normally.
A light coat of grease on the locking lugs of the bolt will help, too.