So my 2005 Avalanche 1500 has a weird problem. When the engine is up to temp driving around 40 mph, 1500-2000 rpms, it starts almost like misfiring and the rpm needle bounces. Once you get above 45 mph it stops and drives fine. My buddy who is a mechanic, checked plugs, wires, alternator, nothing fixed problem. It’s not throwing any codes. He said it might be transmission issue. Took it to a transmission guy and he couldn’t find anything on that end. Has anything like this happened to anyone? At a loss