SAN JOSE, Calif. — San Jose State had an eight-point lead and a defense that had shut down No. 14 Boise State all day — yet Spartans quarterback Adam Tafralis still felt uneasy as the fourth quarter ticked away.

"You don't count a team like that out," Tafralis said. "They're ranked where they are for a reason. They know how to win. They know how to come back."

He had plenty of reason to worry. With the cool of a seasoned conference champion, the Broncos staged a dynamite fourth-quarter rally that barely kept their unbeaten season and Bowl Championship Series hopes alive.

Jared Zabransky shook off three frustrating quarters and a draining illness to lead two scoring drives in the final minutes, and Anthony Montgomery kicked his third field goal — from 37 yards — as time expired in Boise State's 23-20 victory last night.

Ian Johnson ran for 149 yards and a touchdown for the Broncos (10-0, 6-0 WAC), who stayed with Ohio State, Michigan and Rutgers as the only other unbeaten teams left in Division I-A. With two games left against Utah State and Nevada, the Broncos must win out to have a shot at getting to one of the five BCS bowl games.

For three quarters, Boise State seemed unlikely to remain perfect — but Zabransky coolly led the Broncos through their stiffest test of the season against the much-improved Spartans (6-3, 3-2).

"He's a little sick, but he's fine," coach Chris Petersen said of his senior quarterback, who couldn't speak to reporters after going 14-of-20 for 181 yards in his 30th victory as a starter. "He only has so many games left as a Bronco. He'll be out there. If we don't make those plays he made in our passing game, we're in big trouble."

The Broncos have got themselves into plenty of trouble in recent years at Spartan Stadium: Two years ago, Boise State had to block a field goal as regulation ended in a double-overtime, 56-49 victory.

This time, Boise State faced its first halftime deficit of the season before falling behind 20-12 early in the fourth quarter on consecutive TD passes by Tafralis, who passed for 173 yards and three scores.