Honda Accord '03 Stutter/Shaking?

Hello, so this has been an ongoing problem for a few months, but seems to be getting worse recently. I have an automatic '03 accord (I believe it's an ex?) vin number is 1HGCM566X3A010867. It started off with a slight stutter, similar to the way a car acts when it's running out of gas. It seems to be happening more at lower speeds between 35 and 45 mph. From looking at the tach, it typically happens between 1500 and 2000 rpm. I've taken it to my local auto shop and they have no idea what it could be. There are no check engine lights currently, but about a month after I bought the car it did have a check engine light come on. The code was P-0420. After we cleared that code at auto zone, it hasn't come back on. So far, we've replaced the brakes and pads, spark plugs, cleaned the throttle body, and done an oil change. I have absolutely no idea what's going on. Any help would be greatly appreciated, let me know if you need more info.

Thanks.

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