1999 Yukon getting HOT and blowing cold air at low rpm

I have two cars- Yukon and Mitsubishi. The Mitsu began blowing cold at low rpms. No heat until the rpms pick up. Within two weeks, my Yukon is doing the same thing. Both are running hotter than usual,
esp. the Yukon. Both cars problem free for years. All of the sudden... Conspiracy? WTF!
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It's hard to explain the coincidence unless you maintained both cooling systems recently and did not get the air out of either one. Running hot and blowing cold says low coolant or low flow.
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