1986 century T-type shifting problems

I have a century and it won't shift properly, heres what it's doing. It doesn't slip, it goes into gear perfect,
It starts off fine then to 2nd gear it's ok then as soon as it's about 3 seconds on second gear it's like it almost goes into neutral and the rpm goes up to 3500 then after about 5-6 seconds of that it finally catches 3rd gear, it doesn't clunk or anything, it has a 4 speed floor shift, somthing wierd is when i shift it manually with the floor shift from 1st,2nd,3rd,4th it shifts perfect with no problems what so ever. Does anyone know what the problem might be? i think it might be the shift linkage or the dentent cable, or even the actual shift sensor or shift computer.

Can anybody help me out?? and if it is the linkage where is it???? i can't find it anywhere.