An electrical problem forced NASA to postpone Wednesday's liftoff of the space shuttle Atlantis yet again, and engineers faced with a tight launch schedule struggled to understand the problem. About 11 hours before the scheduled midday launch, engineers discovered that a coolant pump that chills one of the shuttle's three electricity-generating fuel cells was giving an erratic reading. NASA rules say all three fuel cells must be working to launch, and if one fails in orbit, the shuttle must come home promptly. NASA officials met for hours during the afternoon to figure out whether they could fix the problem, whether they could safely ignore it, or whether they would have to put the flight on hold for perhaps weeks. The space agency said that if the problem could be fixed overnight or is deemed not serious, NASA could make another launch attempt at midday Thursday or Friday. If not, the space agency may have to wait until late October - or relax daylight launching rules instituted after the 2003 Columbia accident and try again at the end of September.