I went on a trip for 3 weeks, leaving my car parked in the underground parking lot. The first time I used it, the engine was running rough, without any special noises. Just a lot of vibration and shaking while idling. It gets much better once the car starts moving. This has been going on now for 2 months, and it is definitely worse in the morning than in the evening (although it depends on the days). I have no idea what this could be. Spark plugs were changed 2 years ago and they are rated for 100k miles. I only use 91 grade gasoline. The car is a 6-cylinder and has about 150k miles on it. Suggestions?