The hydraulic lifters start to stick with age & the general fix would be to replace them. Though I'm not aware that you have to do anything else regards the cam?
Some people are against this but I did an engine flush during the course of an oil change on my old 90 & added an upper cylinder lube to the new oil & this seemed to cure a permanently stuck lifter. Best to avoid these engine flushes if servicing has been patchy or non existant so they say.
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