1995 Crown Vic no electical power

I have a 1995 Crown Vic that has sat around for the last 2 years. It has started perfect everytime I've tried to started it. About 4 weeks ago it was dead . Tested the battery it was bad. Put a new battery in
and changed the battery terminals also. It started prefect again. Yesterday I sold the car. The guy drove it home and back today to pick- up the paper work. In my driveway the guy went to crank it, it just engaged the starter then went dead. I tested the battery its fine. I took the starter off and had it tested its fine. I followed all the main wiring and it all seems to be fine. Checked all the fuses they seem to be fine. Also changed out the starter relay, still nothing. What could cause this?
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Did you replace just the terminals ? I have more faith in cable sets.
Bill
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