Replace block with a max of 28 teeth and put your NR back on. I faced the same problem with my Colnago when I first aquired it. I was the problem. I rode my 97 RockHopper as a commuter for a 1000 miles then got back on the Colnago with 43/28 lowest and it worked out just fine. I remember Sputnik and saw it go across the sky!