2000 Toyota Camry Engine light

Hi, I have a 2000 Toyota camry, and the other day the check engine light came on, so I took it to a mechanic, they said it was my spark plug wires, so they fixed them and I was on my way. Two days later the check engine light came on again, so I took it in for the second time, they said the sparks plugs needed to be changed, even though I had just had them replaced 4 months ago. So, they changed them and off I went again. The next day the check engine light came on again, so for the third time I took it in and they said thay can't find what is wrong. The car run great, but one small problem when I try to start it after it has been sitting over night or for a long period of time, it starts weird, it takes a minute for it to acctually start. Could this be linked to the check engine light? What should I do? Because my mechanic is telling me, when it doesn't start right up it has to do with the battery. And they checked all that stuff like the battery and the alternator, and they said everything is fine. I am at my wits end with my mechanic. Can you help me fix this problem at all?