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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
The coolant temp sensor could possibly just have a poor response range in it now and with the higher normaly operating temps you pulled up past it when it was warmed up. I would add my vote to changing the sensor.