I have a very similar issue with a '56 hydraulic lifter 235 in my '53 3100. Engine runs great, no smoke. Slight tick at idle. When I do extended high rpm's down the highway , approx 5 min, the tick will get extremely loud and I have to slow down or pull over and stop for the loud tick to go away. If I do high rpm's for a short time there is no problem. I have had the lifters out and took each one apart and thoroughly cleaned them, same prob. I then took them all out again and vented them as per Dave's blog, still same prob. So I took them out a third time and opened up the vent slots a bit more, thinking that they may have gotten clogged with a chunk of crap. I can now fit a .035 wire thru the vent slot, still same problem. Everything about the lifters seems to be in order. I did a quick bleed down check on each with a lever and they all seemed to take about 20-30 seconds to collapse. I am stumped.
