Strange downshift--second time

[funair8594]

I think I've seen this here before, but couldnt find it with the search function. This is only the second time this has happened since I've owned the Z. The last time was about a year ago. The Z has 87k on it--I've owned it since 55K
I was driving in traffic at about 25 mph. Usually, when you floor it, then trans downshifts to 1 and you hit 45mph. This time it went into "neutral", then engine revved to 7K and hit the limiter Took my foot off the gas then it shifted.
Now this was the second of this type of downshift within 30 seconds apart. Possibly, since the fluid was hot from the other downshift, it didnt have the viscosity or pressure at the time to engage the 1st clutch pack? I've checked the fluid and its "good" (using the look and smell method lol) and full. Also I have been getting the soft code 1640, QDM voltage high. Still can't figure that one out. Ive tried cleaning connections, making sure wires were good etc.
This is only the second time. This happened yesterday and it hasnt done it since, and I've actually tried to replicate the problem.
Thanks!

[TurboGTU]

Could be the 1-2 & Reverse Servo Boost Valve. They may have worn and are leaking pressure. But more than likely...I think it's the 1-2 shift valve. They are made of raw aluminum and can stick, wear and side load. the replacement valves and kits I found have hard-anodized valves. That should cure the problem.