Sounds like the lifter isn't getting enough oil to keep it full. A collapsed lifter is not always bad lifter, but usually just one that has varnish and whatever other deposits inside that cause it to stick if compressed further that it sees normal use.

I know when I get stuff from a machine shop it always needs cleaning. Maybe something is blocking oil flow? Or maybe somehow you are losing oil pressure up there from a leak?
When you pull the valve cover is everything covered in oil? Have you ever ran it with the valve cover off to see what is going on? Remember too that oiling problems are often are different when the oil is cold vs 200+ degrees after driving. So a topside leak might be ok when it's cold, but as the oil gets hotter and thins it will loose pressure.