extremely soft 2nd to 3rd shift

I've got a freshly re-built non lock up A-518 transmission that up untill now has been performing flawlessely, But the other day i decided to take my 1 inch carburetor spacer off of my engine and when i did i noticed my 2nd to 3rd gear shift was extremely soft, coyuld barely feel it shifting into 3rd gear :confused:. Thinking it was the throttle valve cable adjustment i re-adjusted it several different times, and nothing i did changed howe soft it shifted into 3rd gear all it did was change when it shifted into 3rd gear.
So last night i decided to put my spacer back on just to see if that would do anything and it didnt :confused:, it still shifts into 3rd gear very soft.
Whats going on here? The 1st to 2nd gear shift is great, nice and firm but when it shifts into 3rd you can barely feel it.
This transmission still has 8 months left on its warranty, so i need to figure this out soon so i can bring it back to the transmission shop fior repair if need be, i just dont understand what could have happened here, it was shifting great for months and then all a sudden out of no where its shifting into 3rd gear extremely sloppy.