When grass dies in patches we need to refer back to what can kill a single grass plant. Cutting off the top will not discourage it, we do that all the time with mowing, but cutting off the bottom, that is, the root, is a totally different matter. There are grubs that can physically chew off the roots, but we should also bear in mind that the soil that lawns grow in is rarely perfectly similar from one patch to another and also that the mix of lawn grasses is rarely uniform in the same way.
Extract a small plug of grass including roots down to a couple of inches from good green and dead areas and compare what you get. Check that the soil types are the same, that is same mix of loam, sand and clay with humus mixed in. Check for stoniness of the soil as you drive in the core tube. Check that grass roots are present and penetrating soil to the same depth. And of course watch for signs of grub activity.
You may find a pattern, soil draining too fast or too slow, presence of grubs, roots chewed, different mix of grasses.