2002 Mazda 626

I have several things wrong and I know nothing about cars. Help!

1) Car is making a sqealing sound. No one seems to know what it is. I know enough to know that it is one of the belts, but every mechanic that I take it to says they can't locate the sound. It does go away or get less "squealy" when I turn off the AC.

2) My check engine light has been on for about 2 years now. This is probably related to my next problem but I am not sure as the next problem started last year way after the check engine light came on. With the check engine light, it would come on for a while then turn off for a week or two, then come back on. Now, it stays on all of the time and sometimes flashes.

3) Car has been "sputtering" as if it isn't getting enough gas when I am pulling away from a light. Once the car gets going, it is fine, but if I stop or slow down, then hit the gas, it sputters, then kicks in.

I hope someone can help me with one or all of my issues. Thanks in advance.