Slipping after rebuild


R.I.P. Lethal GN
OK... Well, my tranny finally died on me recently. i took it to a guy I was told was fast, quick, and did good work. Well after being told it would be done in 3 days, I finally got it back more than 3 weeks later. I couldn't test it at first due to the heavy downpour of rain. The next day I washed and waxed her, and a buddy came over to race me with his 86 GN. Well, I noticed a problem the first time i punched it. It wouldn't bark second even though he drilled out some holes bigger for firmer shifts. After several attempts to see what it has, I found out the tranny wouldn't hold the power. It seemed to have flair-up and slipped through the gears. When building up boost in 1st it wouldn't even break loose. It got to a certain point and jusy held until it shifted. It is a BQ code tranny out of my 85 GN and he said it had all the stuff it needed to holod up in my car... ie servo, 10 vane pump..etc.. The tranny shifts fine and firm under part throttle. It even catches second at times. I have played with the tv cable with mixed results but none being good when it came to WOT performance. I called the guy and he is stumped. He has been doing this for years. What in the world could be my problem?
The list is from here to the floor. Is it only under WOT? Could be cavatation. There are 800 things that "it" could be and drilling holes bigger doesn't neccassarily mean it is better.

When I drive it normally it shifts nice and firm. It almost catches second everytime. when I get on it though, it seems to not be able to hold the power. I'm only pushing between 300-350 through it and everything in the tranny is new. Could it be a pressure problem or a stuck valve or something?
Could very well be pressure related.

Best way to find out is by performing a line pressure test. Your builder should be able to help out there. Also, are you sure that the fluid level is adequate?