Very occasionally, on a full throttle standing start, I've had something odd happen when it should shift from 1-2. It first happened on the first full throttle pull after we put the rebuilt motor back in, and I chalked it up to the computer learning the new behavior of the engine. However, it just did it a couple times today (3000 miles later), and it was repeatable for one time.
Here's what it did today. It pulls very hard through first right up to 6000 RPM, and at the exact moment it would normally do a shift, it suddenly loses power, drops to about 5000, and pulses quickly between 5000 and 5500 like a rev limiter is limiting fuel pressure or ignition. If I back off from the floor just the slightest bit it immediately drops into second and begins pulling hard as normal.
It's only done this a few times, and only since the motor rebuild. Tonight, after the first time it did it, I immediately made another full throttle pull and it did the exact same thing. At 90% throttle it made a totally normal shift into second every time (at about 5500 RPM). After I shut the car off and then started it back up to try again, it didn't repeat, even on a full throttle pull.
One thing I noticed after putting the engine back together, is that the 1-2 shift seems to happen at 6000 RPM on a full throttle run, where I remember it happening about 5500 before. I can't be totally sure, though, where it used to shift. It's 6000 now in any case, which is the hard redline on the tach, of course.
Here's some background: 2005 GTP. Transmission was fully gone through 45,000 miles ago with all new soft parts, pump and converter, and has been perfect since. The engine was recently gone through for a freshening up, which included a new cam. The cam was a Sealed Power "direct replacement" (though, according to Federal-Mogul, isn't an exact factory grind, but is designed to not require new fuel and timing curves or operate incorrectly with factory programming of the PCM while increasing performance). I'm not sure what that means in terms of how far off factory grind it is, but it does seem to rev a bit freer with the new cam. It also got a 3" ZZP catted downpipe at the time, so I'm not sure what contributed to the slightly quicker revving. For all practical purposes, it's basically stock, but like new.