So I just wend out driving to try & nail down some specifics about the problem I've been having. The basic gist is that my engine craps a brick or something anytime it sees 5500 RPM.

here's the truck:
'99NBS Sierra 1500 RCSB 4x4
stock 5 speed with an '01 5.3 LM7
AirAid CAI, 3" in / dual 2.25" out chambered muffler
BlackBear 93 tune

this problem happened twice - both times under the same conditions, so i decided to go out on a test drive & bravely try to replicate the problem again so i could get some specifics. here's what happened this time:

I pulled hard in 1st to 5000 with no problem; chirped 2nd & pulled to 5500, and then (with it still in gear) the truck took a literal nose dive as the engine tried to back itself back down. I immediately hit the clutch & the RPMs hovered at 1000. Then whenever I gave it any throttle, the engine speed would go up to 2000 RPM & back down to 1000 continually once a second or so for as long as the gas pedal was depressed. I then turned the engine off & restarted it and kept driving with no problems. As I was approaching a red light, I decided to give it a rev in neutral. A quick rev to 6000 RPM seemed to be alright - it went right back to idle without a hiccup. Then the light turned green, and as I started to pull through the intersection, it got to what sounded like about 2000 (I was making a left turn and didn't expect anything, so I wasn't paying attention) & then tried to fall off again. Once again, I quickly hit the clutch & it sounded like it was gonna do the up-down again when I gave it some gas, so I quickly restarted the truck & continued through the intersection.

I hope this helps find a solution because I ain't got any clue how people can talk about shifting above 6000 all day (even some manual guys) & I can't get above 5500 without the engine going into a coma. Please help me fix this.