The flat line of the power curve persisted, and there weren't many variables left untested. We wondered if after 157,000 miles, the original valvesprings might be giving up. Not hard to believe, especially considering factory hydraulic roller lifters are notoriously heavy. That puts more stress on the valvesprings, and now that we're trying to rev past the factory power peak, the springs might not be able to control the valve action. It was decided that, even if this wasn't the real problem, a fresh set of valvesprings was definitely a decent idea. We opted to upgrade with Comp Cams beehive springs, which offer improved valve control while also providing additional retainer-to-guide clearance and resistance to coil-bind.