I'm looking into buying a blazer with a sm465 in it but the buyer says that it pops out of third gear. I've heard of this,but what is the cause of it?

I've been told that it has somthing to do with a snap ring that holds third gear on the main shaft.The snap ring gets worn and pushes sidway s some and allows the gears to come unmeshed. I havent personally checked it all out yet, but I have one in my jeep that does it to the point I cant stand to drive it on the highway... I get figure that one out when the ole 350 pukes, till then I'll just live with it.