Na entered the third round leading by six, but he went 3 over par on the back nine at Empire Lakes Golf Course to end the day at 14-under-par 199.
Jeff Quinney shot a 6-under 65 to move into second place at 13-under 200.
Deane Pappas had the round of the day, an 8-under 63, and shares third place with Ken Duke (64) and Grant Waite (66) at 12-under 201.
Na, a Rancho Cucamonga native playing on his home course, played the front nine Saturday at 3 under par. But he was done in by a double bogey at the 12th and a bogey at the 15th -- his first real stretch of bad golf this week.
'This game is all about momentum, and I lost it today,' said Na, who is competing for the first time since withdrawing from the PGA TOUR's Shell Houston Open in April.
After equaling the course record with a 62 on Thursday and following up with a 66 on Friday, Na, a PGA TOUR player for the past three seasons, now faces adversity for the first time this week.
But not for the first time this year.
Na, 23, continued making starts on the PGA TOUR after breaking his hand in a car door in January. Consuming medication to dull the pain, he finally decided he was doing more harm than good.
Now, a week before he makes his return to the PGA TOUR at the Frys.com Open, Na is taking his cue from the game's greatest player.
'I'm trying to look at the positive side,' he said. 'A lead is a lead. It shrunk a little bit today, but overall I'm happy with where I'm at. Like Tiger once said, 'I've got the lead, all I've got to do is tie the guy to win.'
That guy Sunday will be Quinney, who currently ranks No. 8 on the Nationwide Tour money list. A win would move him closer to money leader Johnson Wagner, who was toiling further down the leaderboard through three rounds.
'The No. 1 spot is so important because it gets you in The Players Championship and you can make your own schedule,' said Quinney. 'I'm just trying my best to get a win.'
Ryan Armour shot a 7-under 64 Saturday and is alone in sixth place at 11-under 202. Dan Buchner had a 69 and is one shot further back at 203.