Texas Tech - 20-31 (39%)
Oregon St. - 17-31 (35%)
Arkansas - 16-32 (33%)
NC State - 15-31 (33%)
California - 15-35 (29%)
Boston College - 13-34 (28%)
Also, notable P5 schools with > than 50% win percentage v. P5 in last 5 years (including BYU games):
Texas - 27-29 (48%) - including two losses to BYU
Mississippi St. - 21-24 (47%) - including loss to BYU
Arizona - 22-28 (44%) - including loss to BYU
Washington St. 22-29 (43%) - including loss to BYU