Mazda 626 Check Engine Light?

I drive a 2001 Mazda 626. For a while now the check engine light had been on so I took it to Advance Auto and the code that came up on their machine said that my EGR valve needed cleaned or replaced, I didn't take it to get it fixed rght away, so then I started my car one day and it made a really loud squealing noise then the engine light turned off, a couple of days later it came back on and this time when I started my car it hesitated and started driving horrible, it kept feeling like it was going to die. So I took it to get it fixed 2 days ago, and he cleaned the EGR valve and he also told me the reason that my car was squealing was because it was low on freon. Anyways later that night my car started stalling and I thought it was going to die on me and I wasn't going to be able to get home. So I took it to auto zone and the code said "cylinder 2 misfire" and the guy suggested that I have my spark plug wires and spark plugs changed, but I JUST had them replaced about 4 months ago.......so what all can cause a cylinder to misfire repeatedly besides the spark plugs or wires?