car shuts off while driving!!!!!!!!

My 1995 gmc jimmy shuts off after about 20 to 30 minutes of driving sometimes sooner. I have no idea what is wrong. When it happened, i tried to restart and it wouldnt. Sounded like my battery was dying so i replaced it. Ran for a little while then shut off. Took the new battery home, charged it, put it back in and it started and got me the rest of the way home. Thought the alternater wasnt charging battery so i replaced alternater. Still shuts off! Really scary hope someone can help me........

put a meter on the battery. It should read about 12v when not running and about 14.4v when running. If not, check the positive wire on the back of the alt. and make sure it is putting out. If the alt is putting out but it is not reading at the battery check the wire from the batt to the alt, there may be a fusible link burned out. You can also check for continuity between the pos terminal on the batt and the pos terminal on the alt. It is also possible you got a bat alt.