C-6 transmission shifting hard on freeway

[OrangeJuice]

Hello,

I had level 2 shift kit installed in my 76 F-100 with a C-6 transmission. I believe the shift kit is a TransGo. Recently, I've been noticing when driving on the freeway and I put my foot in it, it tends to shift with a hard bang and jolt - and I mean HARD! It's as if a Mack truck has just bumped me from behind and jolts the truck hard. The first time this happened and my girlfriend was riding with me, it scared her and she thought we got rear-ended by someone...lol. Is there a valve or something in there that is not working like it should?

Thank You