Fuel Injector problems of the 1997 GMC Suburban

We have had continuous problems with the fuel injectors. The previous owner had atleast one injector to go out. We had three more go out and then the dealership replaced the other four back in sept. 2001. Sept. 2002 # 7 injector has gone out again. After speaking with a couple of mechanics and a service manager they reported to us that they have had lot of problems with the injectors in the 1997 & some 1998 models, but mid way in 1998 gm changed the type of injectors that they were using, but they have not recalled any of the old ones with the problems.