In the 21st minute, the visitors went ahead. Lee Sharp's corner was attacked by defenders and attackers alike but, when the ball fell loose, Michael Moore reacted smartly to rifle it into the net from seven yards.
The league leaders stormed back and Alex Keddie made a fine block to deny Stewart Kean after Iain Anderson had headed back John Sutton's cross. It was Keddie who conceded the free-kick in the 29th minute which Andy Millen touched aside for Anderson to blast a low shot past Corr from 22 yards.
Saints struggled to impose any authority on their opponents, yet they went ahead two minutes into the second half. An Ian Maxwell throw was knocked down to Lappin who, from the edge of the box, hit an unstoppable drive into Corr's top corner.
Kean diverted a Charlie Adam shot but Corr saved, and David Hamilton made a superb block on the on-loan Ranger. Moore headed narrowly over, a Bryan Gilfillan shot was equally close and Sharp wasted a splendid chance to equalise as Stranraer showed a rare attacking instinct.
Four minutes from time Keddie impeded Sutton and the striker took the kick himself, beating Corr with a powerful finish. Lappin struck the bar as Saints eased towards three points.