1999 Buick Century bucking at high speeds

this car starts and idles fine and runs smooth at low to moderate speeds. (0-55) when reacing speeds 60 mph and up and mostly when going on an upgrade or up a hill the car sometimes bucks and sputters. sometimes it also runs smoothe at high speeds from 60 -75. the problem seems to be getting worse. when going up hill at speeds of a steady 50 to 60 it runs ok. most problems of bucking are at speeds from 60 to 75.