Rep Power: 684

My car, 99 Camaro SS, I think now has a weak tranny. Well, the last couple times I took my car to 100mph, around 90 when it shifts to 3rd it would lag a second. But this morning it redlined bad and for sure didn't shift till 3 full seconds atleast. Then later, even my my shift to 2nd was lagging it to where I had to let off because it wasn't shifting. WTF! How long do I have till it dies and what will need to be replaced or is there a good rebuild kit I should get?