general problems of the 1999 Toyota Corolla

My car is 9 yrs old, 109,000 miles it's on the lower end of the recommendation mileage for the year of the car. On 9/3/08 first day the check engine light ever came on. I got nervous went to advanced auto parts and they checked the light, it showed something was wrong with the air flow. Check engine light went out after 3 days. Brought it to Toyota for repairs of some other items and mentioned the check engine light going on and then off. They checked it but wasn't sure if it was reading correctly. 4 days later came back on again and went out 3 days later. 2 days passed and then it came back on again and remained on ever since. I brought it back and they told me it was a charcoal canister failure and it would cost $450, I was upset because I just got $968 worth of repairs and maintenance done. I always keep up on my maintenance work/oil changes. Is this a faulty part from Toyota? I've never heard of this going and if it is normal after so many years/miles then why is it so expensive and hard to find or repair? I heard only Toyota has the part which was quoted to me as $232. So if I don't pay $450 my vehicle won't pass inspection/emission test. Can someone help!.

My car is automatic. Sometimes when I am driving it "revs" really high like over 4. And thats only when I am going 30mph. Even if I put overdrive on it still doesnt work. Also my low gas light indicator fades in and out. Its doesnt come on until the car is past the "e" . And the last problem is my locks. When I am driving I lock my doors. And then when I turn off my car, take my keys out, it wont let my unlock my door. I have automatic doors and it shouldnt be doing this. One last comment is that these problems do not happen on all the time. But at least once a week. I have brought it in to the Toyota dealer many times and they say there is nothing wrong. I know there is. Any help would be greatly appreciated. Thank you.