Overnight leader Tyler Williamson managed a 1-over 71 to finish alongside Andy Sanders, Ahmad Bateman, Nolan Henke and Bob Friend at 4-under-par 206.
Cotner began the day in a tie for 11th before making a charge up a crowded leaderboard. He fashioned a remarkable stretch of golf on the front side of the difficult Crestview Country Club layout after thunderstorms halted play for a little over an hour in the morning.
'I didn't know what to expect with the wind,' said Cotner. 'On the driving range it was blowing hard. You just go out there with the mindset of trying to play solid. You keep it in play and see if you can make some putts to make up for the wayward shots.'
The winner of the 2000 Oregon Classic picked up his first birdie of the day at the par-4 fourth after he hit his approach to five feet. At the next hole, Cotner drained a 20-footer to make it two in a row.
Cotner rolled home a putt from the same distance for birdie at the sixth and ended the run with his fourth straight birdie at the par-4 seventh.
'It just seemed like I was seeing the line and the putts went in with perfect speed,' said Cotner.
Cotner cooled before making the turn and ran into trouble with a bogey at the 10th. He parred the remaining holes but his challengers couldn't take advantage.
Henke, a three-time winner on the PGA Tour, stood at 6-under with the 18th hole remaining. He struggled through the rough en route to a double bogey that left him one shot off the lead.
Williamson was at 6-under through 14 holes but also suffered a double bogey after his second shot found the water at the par-4 15th.
'The ball was sitting down so low but I only had 75 yards to clear the water,' said Williamson. 'It came out like a wounded duck. It was right in the middle of the pond. In hindsight, chip out and the worst you make is bogey. It's one of those decisions you've got to make and I made the wrong one.'
Emlyn Aubrey posted a round of 67 to finish two shots off the lead at 3-under-par 207 with Jeff Freeman and first-round leader Barry Cheesman.
Scott Petersen, D.J. Brigman, Tom Carter and Jeff Klauk were tied for 10th at 2-under-par 208.
Casey Martin, who was one stroke off the lead after round one, carded a 69 to finish 54 holes four shots back at 1-under par 209. He was in a group that included Patrick Moore, who is seeking his third victory of the season and an automatic promotion to the PGA Tour.
Full-field scores from Wichita Open