Have you done any basic checks? Scan for codes, check fluid level/condition. If you can pull the shift adapts. Look for adapts above 10 PSi for the given shift you are having issues with. Adapts over 10psi are mostly due to internal issues, but can be caused by pressure control solenoid slow/weak pressure rise also. The adapts are the transmission controller raising base line press by the adapt number to try and eliminate slippage. Also make sure your MAF is clean, it is the main load bearing sensor for the PCM/TCM. A dirty MAF has caused many transmission complaints.
CAI intake, RX catch can, Nitrous Outlet Plate system, APEX air scoop, SOLO HFCs, JacFab ported manifold and upper intake isolator, Solo cat back Mach X system, 3.91 gears, Weld RT-S, 1 piece DSS driveshaft Stall convertor by:CIRCLE D