Try flushing out your Radiator with Preston Radiator flush. The Dexi-cool will crystallize throughout your cooling system, such as the radiator, heater core and water jackets, causing poor circulation.
Once flushed out, make sure your Dexi-cool is a 50/50 mix of coolant and water. To much water will boil over and too much coolant will jell up causing circulation issues again. I know there has been issues with using Dexi-cool from lots of people, but that's just lack of maintenance.
The dexi-cool is rated for 100,000 miles or 5 years before replacement, that still dons not mean that you don't ignore it and and let it to high in concentration of coolant when the water evaporates and then crystallizes.
The only other thing that comes to mind would be a weak water pump that will need to be replaced. Start with the radiator and coolant system flush and try to run may be a 65/45 water to coolant and see if this helps. Just keep an eye on your coolant level once a week, water is the best heat transfer for keeping your engine from over heating. The bad thing is that it evaporates and steams up, so the Dexi-cool brings the boiling point higher to keep it from boiling over.
Hope this helps and keep us posted, just make sure that in the winter you have 50/50 mix or close to it. The water will expand when it freezes and will crack the block and many other parts cooled by the coolant.