Sorry, but that sounds like the behavior that I got out of my cast iron PG in the '57 Chev when I was younger. Girlfriends hated it, and it was due to some internal problem or other. Can't help more than that. Done right, the aluminum PG is a terrific trans for drag racing, but you need a wide power band and/or low gears. And lots of revs.

Valve body problems, maybe? That delayed reaction just doesn't sound right.