Hope you didn't just start it up those 2 days, and then shut it off without giving it a chance to get fully warmed-up by driving for 15 minutes or more. Modern engines need to be at full temp for a while, in order to burn off condensation and other nastys. Plus, the hardest thing on an engine is start-up, when much of the oil has drained down over time. And 6 months is a lot of drain time . Not saying that either one of those things caused the engine to run poorly on the third day, but they're things to be concerned about. You didn't mention how many miles on the vehicle, and how it was running before being shut down in August. One thing that comes to mind that might explain this is either condensation in the gas tank or possibly the gas beginning to turn bad after 6 months. You could try putting some gas dryer in the tank to see if that helps. Many folks prefer Seafoam to clean things up, but there are other products that work similarly. If that doesn't work, then you need to start diagnosing what the real problem is.