This is a very interesting letter. I know what it's like when your favorite product suddenly fails to meet your expectations. I agree that an inspection of the bottler in your area is in order. You certainly tried enough ways to get non-flat Mountain Dew, and you can't just be imaging this.

Have you noticed a date on any of these flat sodas? Also, is it more syrupy than it used to be? Maybe there has been a change in formulation in both regular and diet. Not likely, but possible.