My mom has a '95 Toyota Avalon, and it had a similar problem. There was either a small blip of water in the beakes, or power steering (don't remember which) that was causing a loss of vaccum in the engine. It would also hover near 250 rpm while cold, and was a litter better when warmed up. Though it never stalled, it was just annoying. Try having the brake and PS systems flushed. It can't hurt any...