Do you have any type of alarm installed on your Freestar? The keyless entry remote is a factory remote? Or is it from the installed alarm?
I was just looking at the wiring diagram, there are 3 seperate relays controlling the door locks. It's possible that the unlock relay is not working or the smart junction block is not commanding it to work. The smart junction block is a body control module that controls much of the interior electrical functions. I could not locate these relays in the wiring diagram. I'll need to do a little more digging to see if it's integrated into the module or if it is seperate.
I will check later.
I was digging in the service manual and following through the pinpoint tests, it looks like that the relays are integrated into the module. Your problem could be as simple as an open in the wiring, or you can have a faulty smart junction box. It would be best to see if there are any codes stored in the smart junction box and have the appropriate pinpoint test performed. I thought this would be something simple to isolate and repair, but it looks like you will need a Ford specific scantool to do the appropriate testing.
Sorry I could not give you a simple fix,
This is a brief description from the service manual, the problem you have falls under this pinpoint test.
Just looking at the wiring diagrams and the service manual and it looks like the lock and unlock relays are integrated into the smart junction box. The smart junction box is the large fuse panel located at the LH side of the dash, just in front of your LH foot. It may be as simple as a poor electrical connection if it's working intermittently.