2000 Pontiac Bonneville intermintent loss of power

In curise control mode at 70 mph, engine runs ok, feel a blip once in awhile, when climbing a hill there is a loss of power, engine bucks, shift to 3rd gear it helps, but does not eliminate the problem. Set the curise at 55 mph, could hardly make the next hill even shifting to 3rd gear. Seems to run ok on level ground although I do feel a slight cutout.