What's up with my dyno at 5000 rpm?

Need dyno interpretation assistance. Combo: 91 5.0, P heads, GT40 intake, TFS1 cam, P headers, H pipe, Flowmasters, Novi 1000.

With the 6.87" pulley on the crank, and a 3.5" pulley on the head unit, not pulling timing at all (going from 33 total at 3500 to 40 total at 5000 rpm (14 base + 26 advance at 5000 rpm)), detonating at 5200ish:




Installed 8" pulley with underdrive accessories and the cobra water pump pulley, pulling timing starting at 3500 and ending at 5000 (going from 36 total at 3500 to 33 total at 5000 (14 base and 19 advance)), no detonation, and got this:



Is it timing related or belt slip? (or both??) Or tire slip?

The belt was installed last night, had 25 miles on it, and I got it as tight as I could (I use a long screwdriver to tighten the Novi 1000 tensioner, then tighten the retaining nuts. I am not able to use the hole for the 1/2" socket because my fan shroud is in the way, so I have to use the pry bar between the head unit and the tensioner.

I had another run before this one, where the driver felt the car "get loose" on the dyno around 5000 (like the tires were slipping). I couldn't tell if that was the case or not... and a GT500 got on right after and laid down 580 without "slipping."

I'm running the same springs on both runs, from the Crane cams spring kit. They should be able to handle going to 6k. They've only got 500 miles on them. And it's down on power substantially.