Feels like you run out of fuel, but the scantool says you don't. Car falls flat at the upper rpm'*. Won't make more power, even though the rev'* go up and the fuel trims are correct.
Springs are just weak enough (even though they're 5lbs heavier than S2 springs) that when the came lobe turns and the lifter/pushrod drop, the spring can't close the valve quickly enough. This causes valve 'overlap' in a sense.
I've had valve float in varying amounts since I installed the Jet PCM 3 years ago or so. Only at the top of 2nd gear and 3rd gear. Much more noticable in 3rd though.
The interesting thing is that we found yet another commonality. Buick Grand Nationals have the same springs stock from the factory, and the same install (closed) height. John Wikoff, jr's3800 and I had already pretty much selected a stronger spring for the S1 L67 by calculations and measurements, then after we'd agreed on a spring, I discovered the commonality with the GN. I searched their forums and found out they use the same CompCams springs we'd selected for ours.
Icing on the cake.
Alot of factors go into selecting valve springs. Spring rate, load at valve close, load at valve open (have to know your cam lift for this), etc. Overdoing your springs puts alot of wear and tear on the valvetrain and timing chain/gears.