Q: 2007 GMC Yukon XL 2500. Intermittent problem with very rough engine and check engine light flashes. After a few minutes when engin

Defective ECMs are so rare that half of all warranty returns (industry-wide), once tested in a lab, turn out to be perfectly good. ECM updates (software) are another matter though. In the course of diagnosing something like this, it is worth checking for ECM updates and that can be done by viewing your present calibration (on a scan tool) versus the latest available. If your current operating version is not the latest it is harmless to update. If the problem is intermittent, that would point toward something electrical such as the coolant temperature sensor. When the engine is cold, the PCM has to "know" that to enrich the mixture (to avoid rough idle) and the PCM gets that input from that sensor. Another possibility is a faulty idle air control valve. A diagnostic should include a look at the secondary firing patterns in all cylinders on an oscilloscope during these incidents as that will help the technician narrow the focus if a fuel versus an ignition related fault is the cause. If you desire that this problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a rough idle diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.