OK, now I'm not convinced the MAF is the problem. I just drove the car again and noticed that the flat spot seems related to the position of the gas pedal more than anything. I can induce the flat spot even at a higher speed and higher RPM if I slightly press down on the gas pedal. If I just stomp hard on the gas pedal quickly enough, I can bypass the flat spot altogether. This applies even from a standstill. If I try to accelerate normally or slowly, I get the flat spot and a backfire or two and have to quickly let up on the gas. But if I stomp on it from the light and don't allow the flat spot to set in, I can skip the flat spot altogether and accelerate briskly.

I looked at my last gas receipt and somehow put 89 in there (not premium). Can that cause these symptoms? In any event I was 1/2 tank empty so I filled up with 93 and nothing has changed.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280