2003 Subaru Outback

I bought my 2003 Subaru Outback used about a year ago and it ran great until last June (30,000 miles). One dayI tried to fuel it and found that I could only put a quarter of a gallon before the pump would click off. I tried pulling the fuel nozzle in and out, as well as turning it upside down. After going to numerous gas stations, I simply couldn't put gas into it. I took the car into the service department at a local dealership seven times. Each time they would replace a part in the fuel system (filler tube, solenoid valve and numerous other things I've never heard of). After the eighth time I had it in, they final fixed the fueling problem by "taking the system apart and blowing it out". I could fuel with no problem after this, but now it has trouble starting when the weather gets warmer. I also hear a knocking noise when I accelerate up hill and the car hesitates to accelerate from a stopped position. Recently when I had the oil changed, I mentioned it to the mechanic. He did a diagnostic and found that the sparkplugs (recently replace by the dealership) were not firing properly and showed me how they were burnt on one side. Upon his recommendation I took the car to a different dealership with the information he gave me, they reprogrammed a few things, regapped the sparkplugs and said everything should be fine. However, the car still is having problems, and with the weather getting warmer, I fear the fueling problem may return. I know nothing about cars and I am very frustrated. Any suggestions as to what is going on?