John Lackey put it succinctly when asked why he is now pitching like an ace. ''I'm healthy,'' he said. Then, to remind us all that he is the same guy who won a clinching World Series game, ''I've been locating the ball pretty good, but it has happened before, you know.'' Lackey hurled eight stellar innings last night, propelling the Red Sox to a 4-1 win over the San Diego Padres. One again, the unlikely offensive hero was Brandon Snyder—one of John Farrell's emergency third base options. Snyder capped a terrific at-bat with a bases-clearing three-run double that set the tone for the game. Jacoby Ellsbury extended his hitting streak to 12, David Ortiz hit his 500th career double and Jose Iglesias got two more hits (now hitting .415). On top of everything, the Sox picked up a full game on the Orioles.