Answer: It's normal for the oldest leaves to yellow and fall, especially those on the inside of the plant. Overwatering can cause symptoms of stress, but such stress will appear all over the plant, not just in the center. As long as it is putting out healthy new growth, the yellowing and dropping foliage shouldn't cause alarm.
Watering every other day might be excessive for your potato vine. I'd water only twice a week in the hottest weather (especially if you have sandy soil), and only once a week during the spring and fall months.
Best wishes with your garden!
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