Problems with the DPF often start after the exhaust gas recirculation valve (EGR) starts failing. I would check if the valve is working properly. They like to get jammed with soot.
Just like you said, you should be getting 200+ miles between regens, even when driving mostly in the city.
Just like you said, you should be getting 200+ miles between regens, even when driving mostly in the city.

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