if it were the intermediate band you would upshift to 3rd at about 35- 40 mph and feel like you are freewheeling in between . it sounds like a gov solenoid and sensor problem , get a scanner on it and look at sensor readings instead of throwing parts at it
70 cuda 440 6bbl Auto with a dana 60 68 road runner QQ1 383 auto 1970 6 barrel 4 speed Superbird FY1 number matching 15 Challenger 6.4 SRT pearl White 6 Speed 72 Cuda 440 5 speed 71 Road Runner 440 4 speed