sound like its staying in low gear. on a 400 if you put it in low gear and dont shift, it will shift at 6000 rpm by itself. if the modulator goes bad it still should up shift , late but not that late. be sure your passing gear sw. is not ingaged all of the time and be sure the gov. is working.

check the vac. at the modulator and modulator
unplug the kick down sw.
clean the gov. (be sure the piston is free)
clean the valve body