1995 losing altitude at highway speeds

Hi, when i get up to highway speed (60ish) my truck starts to lose power. It will then downshift and i can get it back up to speed. eventually i have to pull over and let it cool. This starts after about 15 minutes of driving. It dosen't seem to happen if i keep it around 50mph. i changed the sped senser with no result. 184,000 milea. rebuilt tranny. thanks for any ideas.