After winning the opener of the series Friday and moving two games over .500 for the first time this season, the Cubs dropped the final twowith Kerry Wood and Prior on the moundto fall back to break-even.
The turning point was certainly the fifth inning, when the Astros put a season-high seven runs on the board on a grand slam by Mike Lamb and a three-run homer by Adam Everett.
But the Cubs' offense didn't do Prior many favors, stranding 13 runners and having Astros starter Roy Oswalt on the ropes early on but failing to deliver the knockout blow.
"We're just not operating on all cylinders now," Cubs manager Dusty Baker said.
The Cubs had a 3-1 leadcourtesy of a two-run homer by Derrek Lee in the third and an error by Everett in the fourth, which allowed Prior to score.
But the Astros, who have struggled to score runs all season, put it all together in an unlikely fashion in the fifth.
Prior gave up a leadoff single to catcher Raul Chavez and then got Oswalt to pop out. Willy Taveras and Craig Biggio singled to load the bases for Jeff Bagwell.
Prior threw two fastballs past Bagwell before getting him to chase a breaking ball in the dirt for the second out. He then got ahead of Lamb with two strikes and looked like he was going to get out of the jam.
But Prior's 0-2 pitch hung out over the plate and Lamb jumped on it, lining a home run down the right-field line for his first career grand slam.
"That's my fault, I know better than that leave something 0-2 over enough where he can drive the ball," Prior said. "I was one pitch away from getting out of that jam and we're having a different discussion."
Prior then walked the next two hitters before serving up Everett's home run to complete the seven-run inning.
"I don't know if [Prior] lost focus," Baker said. "I'm sure he was upset, that's probably the first time that happened to him. That was just a bad series of events."
Making it worse was the fact the Cubs should have had a comfortable lead by then against Oswalt, who struggled with a sprained right big toe and labored through five innings.
The Cubs loaded the bases in the second inning with one out, but Prior flied out to right and Jerry Hairston grounded out to third.
After Lee's home run in the third, Aramis Ramirez doubled. Jeromy Burnitz then singled, but Ramirez was thrown out at the plate easily by center fielder Taveras. Todd Hollandsworth then walked, but Michael Barrett grounded out to end that threat.
The Cubs were fortunate to get their run in the fourth. Prior doubled to lead off the inning and went to third on Hairston's grounder to second. Neifi Perez then hit a grounder to short. The infield was pulled in, but Everett let the ball play him, backed up a step and had it go under his glove and into the outfield, scoring Prior to make it 3-1.
After the big Houston fifth, Ronny Cedenohitting for Prior to lead off the sixthsingled and Hairston doubled with no outs. But Perez popped out, Corey Patterson struck out on a pitch out of the strike zone and Lee flied out to right.
"We had some opportunities and it just got away from us," Burnitz said.
Even in the ninth, trailing by six, the Cubs loaded the bases with no outs. Ramirez fouled out, and the ageless John Franco came in to retire Burnitz and Hollandsworth.
"If I get out of that situation [in the fifth], there was no doubt in my mind we would have won that game," Prior said. "We were hitting [Oswalt] and hitting him pretty good. If I get out of that jam, I think it's a different ballgame."