Arthur's suggestion need to be stressed. A story best does that. Two days ago, while at my tech's shop, a customer drove in and complained about high temp, etc. We all know the story!
I asked about his 'coolant' mixture ratio. He said ". . I just changed it so it's new and it sb ok". Let's check it.
The handy-dandy Prestone AF tester "pegged" itself to the top! Thus Arthur's point, check your mixture. His was 80/20 and the 80% wasn't water!!!! Nuff said!
Prestone PN is AF1420P. $2.89 at AutoZone.