c-6 3rd gear shift delay

Hello I was wondering if any one has experienced this problem before? I run a 68 mustang with a mild 460 and c-6 trans 2800 stall torque. my problem is, when shifting to 3rd gear there is a terible delay causing my motor to hit the rev limiter. I am able to somehow reduce the delay by getting of the gas. Once the tranny is unloaded it reduces the delay time. The tranny seems to work fine otherwise , no slipping. Both ways = big delay in E.T. the mustang has gone best 10.06 @136 mph. I'm sure that 9's are attainable if I resolve this problem. Any help is good.

Who's VB ?
Frank