Barney, I had a 91 convertible as my buick company car , I was able to get rid of the cowl shake with one change, I had one of my dealers swap the 15" wheels and wheels off of a 90 convertible and it got rid of the shake. I had the service department balance the wheels first before transfedrring them. My 1990 convertible that I currently have ran into somewhat the same type of problem . a number of years ago i bought a set of 17" Lucerne chrome wheels and tires that were new take offs. They looked beautiful on the car but they rode like crap. I stood alongside the tech that I had road force balance those wheels and tires. they still rode terrible. I hated the way the stock aluminum wheel looked and I even told Ed Mertz at one of our meetings they don't enhance the looks of the car. I went back to the stock wheels , had them polished now the car rides great again. Frank