wont stay running very long

Get the charging system and batttery checked out. If you have an electrical meter you could check the alternator and battery. It sounds like a charging system problem, maybe the alternator is faulty. If the battery takes a charge from a good alternater it's probably OK but check the alternator first. If you want a Ford specialist, here's a directory link: http://repairpal.com/mercury-repair-in-elkview-west-virginia

your alternator has gone bad and is no longer chargeing your battery..get a new or rebuilt alternator..and buy a new battery and you'll be good to go..avoid listening to stereo for long periods of time with engine switched off and leaveing lights on..