I've got an 05 GTO LS2-T56 setup in a second gen Camaro that's got an odd recurring issue. When the car is started after sitting for more than about 10 seconds it has limited/slow throttle response almost as if it's in limp mode; the throttle just reacts very slow. If your turn the key on then off and back on again immediately it's fine and if you start it then go key off then start it again it's fine. We've tried two
different LS2 throttle bodies with no change and it's running a C6 Corvette Pedal Assembly.

I cleaned up some grounds and the issue seemed to go away for a while then came back, the grounds on the car are excellent but the issue came back. I installed a better battery in the car and it went away for a couple of days then came back. The biggest positive change was switching the Yellow ACC power wire on ECU Pin 18 C1 circuit 1843 from the vehicles accessory circuit (no power during crank) to its ignition circuit (power during crank and run) but again its back to needing the key cycled for full throttle response.

I suspect I'm somehow not getting proper voltage into the ECU during crank or key-on and the internal TAC module
is acting up but I'm out of ideas of where to look? The attached info is from a GM diagnosis sheet; it seems to be in some kind of limited throttle mode but without a code it's hard to tell which one!

 

Not sure about the 98-02 fbodies but a common issue in the 93-97's is the ignition switch in the steering column. Also not sure if it's even part of the circuit that's giving you trouble but it's the first thing that popped into my head and your thread has no responses yet haha. shoebox has a good write up on his site about changing it out.

 

Data log it, and take a look????

 

We're having this exact same problem. Did you ever get it fixed?