oops, forgot...
Valve springs not being strong enough to handle that kind of lift as speed will cause valve float and definatly kill power. Who knows about the springs you have, maby upgraded, but maby not upgraded enough. Valve float can usually be heard as well as felt.
Also, Ignition, what plugs, thier condition, and where are they gapped? How about the cap, rotor, wires? It could be possible that the spark is being "blown out" at higher rpm's.