Why would symptoms of hyperglycemia improve 30-60 minutes after eating, allow you to feel normal for several hours, and then have symptoms return?

Hyperglycemia: Unless you are diabetic having "symptoms or hyperglycemia" would be unusual. Unless your blood sugars are remarkably high, says over 300, it is not likely you would notice symptoms. Any fasting BS over 130 is diagnostic of diabetes by most standards. Are you sure you do not mean "hypoglycemia" or LOW blood sugar?

What symptoms are: you referring to, as hyperglycemia is usually asymptomatic unless several times normal. You may be overweight and insulin resistant. Talk w/your doctor and check your actual glucose level rather than "symptoms".