B737-400 suffered loss of auto pressurization control.

Narrative

Climbing through FL 250; cabin altitude warning horn sounded. We donned oxygen masks and followed QRH procedure for Warning Horn - Cabin Altitude. Also placed both pack switches to High; but no success. Per QRH; we accomplished the Excessive Cabin Altitude checklist; and regained control of the pressurization when we switched to Manual AC mode. Cabin altitude was climbing at approximately 500 fpm during this time; and stopped at approximately 12000 feet when we regained control. Cabin altitude descent was approximately 2000 fpm; and was rather abrupt to the Flight Attendants and passengers. No Auto Fail light; nor did the controller automatically switch to standby. We requested a level off at FL250 with ATC at the start of this event; and stayed there during this time while we considered if we wanted to continue on the filed overwater route. We contacted the Dispatcher; who contacted Maintenance; who suggested we try the standby mode. Standby worked fine and we decided to continue but refile our route over land. We continued to destination without further incident. This was a repeat event from several days ago; and maintenance had just removed the MEL prior to this flight. Don't know details of what maintenance actions were accomplished to clear the MEL.