Take merlin2021's advice and get the system pressure tested.
The overflow bottle itself could be the problem. My 97 started losing coolant and the one place I didn't look was the overflow bottle.
Once I pressure tested it, the overflow bottle looked like a lawn sprinkler.
If that's your problem, they're still available at Ford for about $40.
Hope this helps.
Wednesday, November 28th, 2007 AT 9:29 AM