[j cAT]

these pre 2006 GM vehicles now require dexron VI NOT dexron III !!!! also your vehicle and other model years had defective cat converters. what could be wrong is the engine has an intake leak too lean also dirty fuel injectors. does the transmission properly down shift when on hywy speeds ? it may stay in over drive and not drop out causing a drop in speed. you never have a GM transmission flushed this is damaging to the torque converter . the method to use is the FLUID EXCHANGE METHOD . THIS TAKES MORE TIME BUT NO DAMAGE.....