TF 904 Auto slipping 2nd to 3rd when manual shifting @ higher revs

Thanks everyone for your input... I asked several guys here about driving without the kickdown hooked up as I was concerned about driving it without being hooked up and they said it would be fine, you can simply manual shift it (as I often do anyways)... And it seems fine except that actual issue I've described... Can anyone explain what is actually happening/ not operating correctly to cause my issue? I appreciate it may be due to the kickdown not being hooked up and I'm not going to drive it anywhere till it's hooked up, but just curious to know the exact reason it's slipping only when accelerating aggressively VS normal, thanks