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

Hi everyone, so I'm in the middle of hooking up a Lokar type cable kickdown system, and have it connected at the trans, but not yet hooked up at the carb end. Anyways, whilst waiting to get new brackets etc to hook it all up I've been driving my car (68 Dart w 318) around, to test other recent work etc.
I usually shift manually through the gears, and have been doing that now, and all is good if I drive sedately - i.e. rev normally up to 3500/4000 and shift up; Trans shifts nice and smooth as per normal. However, IF I stomp on the gas, in 2nd gear, then go to shift up to 3rd , at say 4500 or higher it slips as if I've shifted into Neutral!... Is this because I don't have the kickdown connected? Or is it something more sinister y'all think?... Welcome any feedback/.insight as I don't really wanna drive it around whilst it's doing this... thanks