what i'm looking at running next year along the same lines as being discussed....looking to turn in the 8 grand area.... but, to cure your valve float, change the springs out to stiffer springs (like the 33's mentioned bove). your hitting the upper popwer band and anything after that is wasted....you need to get the valve train working....i'd start with a good set of 32 or higher valve springs, replace the keepers in case they had worn out, look at the heads of the valves to make sure they haven't mushroomed, check your pushrods for straightness and i'd even crack the block open and look at the cam and lifters to make sure they are wearing..
springs will wear out (no definitive time line or useage)....but they will wear out and get weaker from constantly turning above 5K....you could jsut be suffering from weak springs and that will cause valve float. spring pressure has to push against the rocker, which pushes agaisnt the pushrod and keeps the lifter face in constant contact with the cam lobe.