What is wrong now? "Check fuel cap" light on and "engine" light flashing

Took car into be repaired because of a misfiring. They said due to mileage probably time for a tune up. They also said rear wheel cylinder was leaking so that needed to be replaced and a brake flush was needed. Throttle body was checked and an injector flush and tune up was done. I had just replace the gas cap to months prior but they put on a new one even though I told them I had just done that and didn't need one. Bill was around $3,000. Within a week of getting it back from the shop, it started misfiring again and the "engine" light came on. This time computer said it was Coil #4 was the problem. $300 later picked up the car and as I started it and began to drive it started shaking worse than before. This time computer said the problem was with Coil #2 and Coil #6. Mechanic said these did not show up the last time just like the problem with coil #4 didn't show up the first time. They repaired this for free. I get my car out of the shop within 2 days the "check fuel cap" light is back on and the "engine" light on a day later. Mechanic said probably engine light on because of the fuel cap thing and that is due to vapors escaping (but they had just replaced the fuel cap for $59 and I didn't even need one). NOW a few days later the car is misfiring worse than before, check fuel cap light is on and engine light is now flashing. This has been a nightmare - PLEASE help - I don't know where to go from here or what to do