I got a used Argie from a gun show locally and have really enjoyed it.

It has started to have FTFeeds on just about every other round.  Almost every round from the left side of the magazine causes a Bolt over base type jam.

There is enough gas, it feeds from the other side fine and ejects fine.

I have one or two mags that will run with no problems, but I can't see any difference between them and the mags that jam.

What should I check?

if it's not just the mag, try lightly breaking the edge of the receiver's underside feed rail on the offending side.

Quoted: FAL mags are cheap enough. Pitch it.

Thats the thing, its not consistant. Sometimes a mag will work and some times it wont.

I have had the same Malf out of 6 or 8 different mags, all on the same side