Filament grinded? Remove it, look if the feeder grinded your filament. If so, cut that part off. Nozzle clogged? Clean it. There are some videos by BCN3D on youtube. Overuse of retraction? Disable it (for testing purposes, lower the retraction speed). Since the first layers look fine, you will have the right temperature. There are several threads about that issue with the Sigma. Most effort for that solution is to add some heat-conductive paste. But for that look the how to up.