1995 Ford Windstar runs hot and poor heat

Van has 3.8 ford replacement engine 7 years old and 60000 miles on it. About 6 months ago it was running hot, replaced t-stat, was ok until 2 weeks ago temp. Gauge was irratic and then going into red and poor heat. Replaced t-stat with Stant. It was ok and great heat for about 2 weeks, now back to red zone in 5-10 miles and poor heat. Runs ok, has no missfire and no check engine light. Any ideas?I am going to try running it without t-stat and see what happens. Any thoughts would be greatly appreciated.