Couple times I had a good tune and shifted right where it was set (+ - a few hundred rpm)... when I opened up the exhaust it raised shift points a few hundred RPM = not accurate to what was set in tune. But was responsive to further changes. Gotta agree with Hog but it did change on me and shouldn't?

Sometimes people don't believe a change of plugs will make a change when they only have a couple thousand miles but I've had engines break up at the track at high RPM and a fresh set of plugs cured it.

That said I also have had issues with anything other then stock spark plugs, so I'd start there and because of the fancy Vortec distributor do a cap and rotor if the plugs did not cure the issue.

Hog, I'm leaning towards spark because of knowing his fuel pump etc... was new and had plenty of flow in data logs. But at this point we should be lookking at some WOT data logs!