I am half way through installing my pre-finished 3/4" x 3-1/4 solid white oak hardwood floors. Sub-floor is 5/8" plywood tongue and groove, 16" on center joist (2x8). I used #15 felt, stapling 6" and 3" from edge. I'm using 2" staples. First off I think I had the air pressure set to high for the staple gun. I had pressure set at 95, when the recommended pressure is 80-90. I didn't have too many split tongues, however there were some. Anyways, for the most part when I walk on the floor it feels solid, I do get a few cracks/pops when I walk on the floor, but I'm thinking it is just "settling".
But there is a few planks that moves down about 1/8" and squeaks loud when I walk on it, mostly on the ends, only one board moves on the groove side, but its movement is alot. It would seem that either the tongue broke, or there was a mill problem with the tongue and groove. My questions is:

1) How concerned should I be that I had the pressure set to high? I do get the occasional pop and crack sound when I walk across the whole floor, but not all the time and very little.

2) How can I fix the boards that seems to have a low spot? I can't get to it from underneath, I was thinking about using some glue injection just to fill in the hollow spot below it so it has something to press against, but I heard that was just for glue down installation. Any ideas?

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Is 95 psi the lowest pressure that sets the staples to the correct depth? If so then you're going to have to leave it there or lower it and finish setting the staples by hand. The higher pressure may cause the stapler to wear prematurely.
Try putting some talc in the joints that have the ocasional pop.
When the end of a board sinks down the 1/8" will the end of the board that it abutts also sink down if you step on it at the same time?

The floor does look really nice.
Did the manufacturer's installation instructions indicate that staples were ok to use? I've not used staples but a lot of what I read indicated that tongue splits are more common with staples than cleats. Based on results to date would you consider switching to cleats?

Staples are fine to use. I have been using 95 psi which is sinking the staples too deep, sometimes causing the tongue to split, so I am lowering the pressure to the recommended 80-90 psi. I have never used cleats so I can not comment on that. This is the first hardwood floor I have done and my father had the staple gun so thats why I used staples.