Each lifter establishes a running fit with the cam lobe it's mated with during the first few minutes of running when the engine is new. Mixing up used lifters can cause rapid wear of the lifter and the cam. Check the bottoms of the lifters with a straightedge like a 6" machinist's scale and a bright light. If the bottom of the lifter is flat, or worn concave with light visible at the center, the lifter is worn out. You should be able to rock the straightedge slightly from side to side on a good lifter. If the lifters are worn concave, the cam will have a corresponding wear problem.
The only really effective fix is to install a new or reground cam and new lifters, since even running fresh lifters against a worn cam can cause problems. There have been thousands of words written here and elsewhere about cam break-in procedures, and everybody is passionate about their own particular ideas of what's correct! Use the search feature to find "cam break-in" and get ready to settle down for a day or two of interesting reading!
Jerry
