Does the cable have any slack in it with throttle at the closed position?
If the minimum cable setting is not enough, the trans will go into "fail safe " mode IF the fail safe check ball is installed. This fail safe mode will cause high RPM shifts like WOT and the shifts will be firm.
You may also have a worn out valve bore for the TV valve in the valve body. Oil pressure can get behind the inner TV valve and force it over to MAX stroke... that will also cause late - firm shifts like a WOT
Dont worry how old you are... it is the mileage on your body that adds up.