Tranny shifts early

I have an a-500 in my Dart, it sat for almost 6 yrs after rebuild, it shifts early as in 3rd gear by 20 mph it also won't kick down at any speed I removed oem linkage and installed lokar cable no change also the lever on trans is at halfway with throttle closed will only go back if pushed is the problem in the valve body or somewhere else inside?
Thanks
Bruce