Well it looks like it is the heated windscreen causing the problem:
"The vehicles listed below are equipped with special windshields that contain metal in the glass(metal oxide) that prevents the E-ZPass tag from being read properly. Windshields with solar ray glass, solar tint, heated or heat reflective windshields, and insulated or Insta-Clear glass may also prevent the tag from being read properly.
Standard EquipmentMakeModelYearBuickRoadmasterAllCadillacCatera99-01ChevroletLumina VanPrior to 2002Venture Prior to 2002FordCrown Victoria*1987-1994Taurus*Prior to 1995OldsmobileSilhouettePrior to 2002PontiacTrans SportPrior to 2002MontanaPrior to 2002SubaruSVXAll
This has to be the first time a Seven has been associated with a Buick Roadmaster. oh the shame