You need to decide what you want, a controlled impedance PCB or
 a PCB with controlled dielectrics and trace widths? It should be
one or the other. No fabricator wants their hands tied to meet an
impedance requirement. The fabricator can meet either an
impedance requirement or contolled dielectrics & trace widths.
With the latter, you'll want to specify the specific prepregs, and
cores ( with the prepregs used to build that core) in order for the
PCB to perform electrically the same from fabricator to fabricator.
In addition, you'll nee to specify the resin content of each prepreg
being used. As you can see, it can become very hairy to specify
the controlled dielectrics. Not to mention that, not all fabricators
inventory the same cores and prepregs. Cost is not particularly a
concern when specifying the stack-up, but delivery time can be
from fabricator to fabricator.

How was the impedance verified on the new boards? Are you sure
the impedances are met? A written report isn't always as accurate
as actual TDR measurements. (Can you read between the lines?)

Good luck,

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