I'm not really thinking preload but it could be if the pushrods are a bit too short & the cup is too close to the snap ring at TDC then when things get hot the clearance decreases even further till there is contact making in effect a solid lifter sort of. ~.020 is ballpark. put a lifter on base circle and set a 6" thin steel scale "ruler" across the valve cover rails then make a sharpie mark on the edge & slide it over fast till it touches & makes a witness mark on the side of the pushrod then unbolt the rocker shaft evenly till the pushrod JUST stops moving upward (no sideways movement that skews your test) & this is when the lifter cup has reached the snap ring. make another witness mark then measure the distance between the two witness marks & that is the preload distance. its easier to finish removing the whole rocker assy then so you csan remove the pushrod to make it easier to measure it. On the other idling with valve cover(s) off press straight down on each rocker arm on the lifter end with a wooden hammer handle (hurry it is a messy job).