2004 Dodge Intrepid sometimes car dont start even when it c

For some reason, I bought this car and it worked fine with no problem for 4 month, recently I took the car to a dodge dealer for a full tune up and service, I noticed after the service, the check engine light started coming on and off when ever it feels like, sometimes the check engine like can stay on for days, also noticed that when the check engine light comes on and I go above 2 RPM, my cars starts to jerk, but when the light is not on and I go above 2rpm its drives fine. Now I noticed that sometimes when I try to start the car, it cranks but doesnt start, after a couple of tried (20 or more cranks, ) the car finially starts, some people say I might have water in my gas tank, please could anybody tell me what might be wrong before these mechanics give me a bill way more expensive than the car.