High speed lifter collapse is common and can usually be fixed very easily. It is caused by air being trapped in the lifter. At low speed the oil stays at the bottom of the lifter internal reservoir. At high speeds the oil is thrown to the top of the reservoir which puts the air against the check valve so when the lifter recharges after bleed down, which happens a little every time a valve is opened, it is recharged with air. When the engine is returned to a slower speed the air is bled out and a quiet lifter returns. The problem is the air stays in the reservoir waiting for the next high speed run.
Most aftermarket lifters have a vent hole in the pushrod seat that is sealed off by the pushrod. I grind a very small groove on the bottom of my pushrods to allow air to escape from the lifter reservoir through the vent hole. 1950-52 Power Glide engines were grooved from the factory this way.
