The loss knocked the Bearkats (6-5, 5-3) out of a four-way tie for first in the Southland Conference.
Donovan Williams scored two early touchdowns, and a 23-yard interception return by Dwayne Williams staked Sam Houston to a 21-7 lead after a quarter.
Northwestern State (3-8, 3-5) turned the tables with a 17-point momentum swing in the second quarter. Eppler passed to Quan Shorts for a 78-yard pass play, Daniel Justino added a 43-yard field goal and Shemar Bartholomew returned an interception 96 yards for a 24-21 halftime lead.
The teams traded TD rushes in the third quarter with Eppler running in the game-winner from 5 yards out. The Bearkats were held to four punts and were intercepted on their final five possessions.
Sam Houston State's five losses have all been by a touchdown or less.
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