It's not being installed correctly then. What happens is the lock ring will spin in the hub, slowly machining the slot wider or just getting spit out.
Doesn't help that the TSRM is a bit vague on how it's put together as if it was installed wrong to begin with (like on my car) you have nothing to compare it to.
Now, I can try to help though as best as I can. I'll referance this picture:
Ok, 31231 is the hub. 31264 is the cone washer. 31264A is the flat washer. 31231A is the throwout bearing. 31231B is the lock ring that holds the hub to the bearing.
Now, put the hub on the bench with end you install the throwout bearing on up. The cone washer(31264) is used to keep tension on the snap ring and bearing, so it MUST be positioned correctly. If you place the cone washer on the bench, the outside edge should be raised off the surface. If the center edge is raised, flip it over. Now, put the cone washer on the hub in the same direction you took it off the bench. You then put the flat washer(31264A) on next. The throwout bearing(31231A) then gets put on with the flange on top. Now you install the lock ring(31231B). It's not going to be easy as you have to fight the spring action of the cone washer. There should be no play in the assembly.
Now you can slip the assembly through the back of the pressure plate(31210) and install the flat washer(31231C) and the wave washer (31231D) and hold them in place with the lock ring(31231E). The assembly should be quite stiff. If there is play in any of the assembly front or back (basicly if it would rattle), then replace the washers (namely the wave washer and cone washer).
Also, they're lock rings, not snap rings. The TSRM says to use snap ring pliers, but you'll hate your life using them. You'll either break the snap ring pliers, or they'll slip. Get some lock ring pliers like:
http://www.craftsman.com/shc/s/p_10...ring+pliers&prdNo=11&blockNo=11&blockType=L11
Hope that helps, I really should have taken detailed pics when I did this years ago as it seems to be a common issue.