I have a 03 Suburban LT with the 150,000 miles. and it misses out at 50mph or higher. I thought it was the transmission, and the tranny guy confirmed a new transmission was needed, but said he thought the missing was maybe the coils. the local Chevy dealer did the diagnostic, and determined the coils, plugs, and plug wires were all good. I 've replaced the oxygen sensor as well. The service manager said to keep driving it until the Check Engine light comes on. That concerns me, but hey what do I know. The mileage hasn’t dropped off, and it runs fine the rest of the time.