03 Tiburon really rough idle then stalls

Cars been sitting for 6 months and just got it started today. It was running kinda rough but smoothed out now after running up and down the road for about 10 minutes I was coming to a stop sign and started downshifting.. the car kinda backfired then stalled right out. It would start up briefly and stall within a few seconds but running really rough. When checking under the hood the oil cap had blown off and ended up sitting down in the fan shroud. The oil was just checked prior to driving and was good and clean. Towed the car home but it will not run fired up runs very rough for a couple seconds then stalls you can feel the engine shaking really hard. What do you think guys. The car has 117k and the clutch and timing belt were replaced a thousand miles ago.