At the track last night it would level off and loose power?

Generally, when an engine quits reving, and is not accompanied by misfires, It's either a little short on fuel, or it's not breathing right.The short on fuel has been mentioned.
The not breathing right, is usually a plugged/restricted exhaust, or as mentioned, valve float. When it's valve float or lifter pump up, the rev ceiling is usually the same,or close to the same, no matter which gear you're in. And you can really feel it coming on. And if you drop it into neutral, it usually won't idle, til the lifters bleed down.
When it's restricted exhaust, it usually gets worse with speed as the engine works harder and harder, and you can really feel it coming.
If the secondaries are not opening, it takes a long time to get to the end of the track, but it pulls redline, at least in first and second.
If the carb is really fat, as in choke-stuck-partly-on fat, well, that will kill performance.
If theres 2 qts too much oil in the pan, that might do it.
If the tranny's trying to catch two gears at once, that'd be like driving with the brakes on.
If the points are bouncing, it'll flatten out, but be accompanied by misfires. You're not running points, are you?
If the timing chain jumped a tooth or two, that would do it, but again it won't idle normally, cuz it's all messed up. If you put the timing back, it will run with a tooth or two out, but it won't breath right andwill flatten out real quick.
If the rotor gets to be out of phase with the correct towers, it will flatten out, and be accompanied by missfires. But this usually comes on at a much lower rpm, when the advance gets close to max. Say closer to 2800 to 3600.
A faulty rev-limiter?I have seen cheap ones act weird.
Anyhoo, those are some ideas that came to me.