1995 Honda Odyssey 279,000+ - Check Engine Light

Recently the check engine light has been coming on during my daily commute at the exact same time and spot. After about 5 miles on the interstate the light comes on but I cannot detect any symptoms, the car runs great. This rarely if ever happens city driving, only on the interstate. Can anyone offer me advice as to what I should have checked? The car is too old to be hooked up to the fancy machines for diagnosis.