My 2007 Saturn Aura XR 3

My 2007 Saturn Aura XR 3.6L wont start. It was running and then I stopped at a store, when I came out 5 minutes later the car wouldnt start right away. It theb started but very rough and sputtered. I was able to pull out of the spot I was in and when I attempted to accelerate onto the road it died. I could get it started again but very very brief and it was hard. Now it wont start at all, It cranks but just wont turn over. Changed the plugs, checked compression, checked fuel system all ok. Even chagnged the Crankshaft postion sensor since that was a code that was tripped prior to this. Please help someone. I dont know if it jumped timing or if there is some other problem.