lifters are very temperamental. If they get starved of oil the spring can wear the metal inside. The hardest part is finding the thing, we have 20 to choose from lol
On my old XR2 I put a brand new set in with my fast road cam and ran them in as per the instructions. Within 10 miles one had worn to the point the spring had started to come through and it was spitting fuel back up through the carb

Put a replacement in, never had any trouble with it after that so I put it down to the oil way being blocked