[olddog]

If it is valve float, the springs are not closing the valves, and the lifters will pump up to take out the slack. Lifters do not bleed back down quickly. So if you are making good power at 5400 rpm and it falls on its face at 5500, you need to back down to 5400 and see if it is still making good power. If it was float, the lifters will still be pumped up and it will still be flat. Typically you have to back off the rpm quite a bit for a while to get the lifters to bleed back down. Try dropping 200, 300, 400,,,,, and see how far you have to drop to get the power back. If backing the rpm off a 100 instantly restores power, I doubt the problem is valve float.

I think this simple experiment will shed some light.

I agree that a wide band O2 would be very helpful.