Help troubleshoot 904? Slipping at higher RPMs

Thanks for all the suggestions.

To answer a few of the Questions..

fishy68 - Yes, I checked the fluid in Neutral.

OldmanRick - As for the Shifter linkage maladjusted, I have a new B&M Stealth Shifter in the car and I have checked the Cable adjustment at the Trans a couple times and it appears to me to be properly adjusted (I had this same thought when this first happened as it is so strange and the problem goes away soon as I shift).

TrailBeast - I have a deep Pan with drain on it so it will be easy to change the fluid. I am going to do that next to see how the fluid looks. It looks/smells fine to me on the stick but I am going to Drain it and look for metal ect.. this weekend.

And Yes, It could be the Converter or something else completely. I just don't know how to tell or what to check on that. I had the converter custom built for this car/combo by a reputable builder. It should be about 2800 to 3000 stall. I do know that it seems a good bit tighter than that as the trans engages and moves well at about 1500 RPM in first. I have not been able to really test it by Flashing it to see where it engages yet.

The engine appears to be just fine. Runs/Sounds/Revs great.

And Thanks to all of you for taking the time to respond. It is appreciated. I am still holding out hope that it is something obvious that I overlooked or an easy adjustment.