727 B&M shift kit problem

Did this problem ever get resolved? I have a 1968 plymouth satellite 383 727 combo that does exactly the same thing. When manually shifting the transmissin the trans feels like it flairs or maybe gets stuck between 1 and 2nd . If you lift out of the the throttle it shift into 2nd and you are fine to do a wide open 2 to 3 shift. If you just leave it in drive, It shifts 1st to 2nd just fine and actually breaks the tires loose during the shift.
The motor is a stock bottom end 383 with a mopar purple .509 lift cam, holley street dominater manifold, edelbrock 625 carberator, headers, and mallory dual point distributer. It has a 3000 rpm stall conveter with a 3.9 sure grip rear axle. shifter is in good adjustment , I am considering going to a Turbo Action Manual valve body. I have a more potent motor on the stand that I am getting ready to put into it in the next couple of weeks. I want to make sure the trans will hold it. The new motor is similar to the old one except the 906 heads are ported and the pistons are forged 11 to 1 pieces. It is another 1968 383. I am also considering giving it a shot of nitrous.