ARLINGTON — The St. Louis Rams were the only team in the NFL to go through the first two weeks without allowing a sack.That streak didn’t last long Sunday afternoon at AT&T Stadium.Less than 3 minutes into the game, Sam Bradford was wrapped up and taken down by a blitzing Orlando Scandrick. It didn’t get any better for St. Louis in a bumbling 31-7 loss to the Dallas Cowboys.“You name it as far as execution,” Bradford said of the Rams’ woes. “Whatever it was, we didn’t do it. In the first half, we played behind the chains. It was a very poor performance.”The Rams were awful on third down, couldn’t move the ball through the air or on the ground, and were riddled defensively.St. Louis coach Jeff Fisher was baffled by the showing, especially coming off what he called a good week of practice.“We didn’t really do anything well today,” Fisher said. “We didn’t run. We didn’t stop the run. We didn’t throw it or stop their passing game. We had difficulty on third down, both sides of the ball. We obviously had some penalties on special teams.“They’re a good football team. I didn’t see this coming. This is a defining moment for us right now. We need to get past this.”St. Louis was 1 of 13 on third down, with its only pickup coming on the last play of the game. The Rams didn’t gain a first down until less than 9 minutes remained in the second quarter, and that was their only first down of the half.St. Louis had only 18 yards of offense in the first half, compared to 202 by the Cowboys. Bradford was sacked six times, including four in the first half. Three of those first-half sacks came on third down.Even when something went right, disaster followed. Bradford’s 26-yard completion to Chris Givens early in the third quarter — their longest play to that point — ended with a lost fumble. Tavon Austin’s 84-yard punt return for a touchdown was nullified by a penalty.Can’t stop MurrayThe Rams heard all of the talk out of Valley Ranch this week about the Cowboys’ struggles in the running game.DeMarco Murray finished with 175 yards and a touchdown.“He’s a hell of a player and if you don’t execute against a guy like that, he’s going to test the edge of the defense,” defensive end Chris Long said.