There should be a tv cable from trans to throttle body, if this is adjusted too tightly it can cause a late shift or sensitive kickdown. A missing return spring from the TV lever at the trans can also cause trouble like this. Other possibilities can be the Throttle position sensor out of range or even a leaking governor valve circuit in the valve body or bad governor pressure sensor/solenoid.
The governor pressure sensor/solenoid is the one I was gonna start with. The the tv cable sounds like a good place to start tho! Iv never heard of the leaking governor valve circuit. What and where is it?