Do you run a windage tray? My old Man had a problem with a big block he raced, in the upper RPM's the oil clung to the crank and the oil pump would scavange. Keep in mind this was a serious race only engine and the RPM was near 7500. He installed a windage tray and baffled oil pan, corrected that problem. What I have trouble with is the slack in the push rods, normally you set them when cold by pre-loading the lifers, if the lifters are pre-loaded where does the slack come from. I have broken valve springs as well and it could be felt across the entire RPM range. True they may not be strong enough, but why would it only show up once the engine warmed up. You have a unique problem.
