What is causing my St. Augustine grass lawn to yellow and die in patches that are in the sun on good well drained soil?

The problem is likely from the feeding activity of Chinch Bugs. Chinch Bugs are very small insects that you are not likely to see without closely inspecting the grass. Dig up a section of the declining grass and shake it hard over a sheet of newspaper. If Chinch Bugs are present you should be able to see them rapidly move about on the paper. You can also bring us a sample of your lawn for inspection. To control Chinch Bugs we suggest spraying with Acephate 75 and spreading Turf Ranger granules 2 weeks later