Sounds like you're on the right path with thermostat. A quick search gives a Motorcraft sourced part #RT1157... for $120!!!!!
http://www.partsamerica.com/ProductD...tNumber=RT1157
Holy cow, what ever happened to a simple $4 part???!!! What a wallet-drainer, good luck with the replacement.
I'd go for a good system flush and fresh coolant while you're at it.