My car has been having a low idle when I stop at lights.

What is your question?

Car has 245,000 miles but I have always taken good care of car. No check engine light has come on. I took it to a mechanic and he said it was bad spark plugs due to bad gas so he changed them. Car was okay for a little bit and then I had the same problem again. Took car back to mechanic and he said it was now water on the spark plugs so he changed again. Car was okay for a little now I am having the same problem again and now it is worse. There is also hesitation upon accelerating. I think the problem is more than just bad spark plugs. I even changed the gas station I go to. A new Chevron gas station and I use 91 gas all the time. But the problem persists.