If the valve clearanced keeps increasing something has to be going on, but it doesn't have to be the lifter. The lifter is just a piece of hardened metal, so if it is hardened properly there isn't much to go wrong. A more likely culprit is valve seat recession. This is where the valve seat is loose in the casting and the constant pounding wears the seat deeper into the casting. Mark A had a thread on this a while back. I have never seen it happen on a F/2F because I haven't seen that many, but it was pretty common on air cooled VW engines with aluminum heads.