I'm with Rumblefish on this one..
Check your lifters. Inexpensive lifters (cheap check valves) won't hold higher spring pressures.
Back in the day, we used to trade out the plungers and valves from a lifter brand that didn't use them in a 904 lifter.
Solved the problem.
Manufacturers caught on and started implementing this strategy, but at a cost. Well worth it.
