Dynamic396, thanks for your input. Hand's-on tech experience is a valuable thing to have.
I am inclined to think that the stacking bearing issue isn't my case here as it only happens once it is hot, and even then - only after running for a while. I have just bought and thoroughly cleaned a new set of GMPP lifters, and I'll be slapping them in later.
Out of morbid curiosity, I took a can of brake-cleaner to the lifter pictured above. It had been sitting out on its top all night and the oil had all drained out of it. I stuck the tube in the oil bleed hole and gave it a squirt and it was completely plugged up. It took a while to blow through whatever was in there, so I think I've found my ticking lifter. Time will tell, though.
Those bearings may just get me yet.