Engine Light

Hey there:

I own a 1998 Ford Windstar Northwoods Edition Minvan, which I bought in August of 05'. It seems that everytime I put gas in my van, a month or so later the engine light comes on, So I go to a machanic and he tells me it's a faulty gas cap. Yes I bought a new one (Gas Cap) and the problem is still there.
I have paid over 100 dollars for these machanics to take the light off (20 dollars each visit), but the problem persists. What the heck can I do?
I can't seem to get a response from Ford and I looked to see if it was a recall, Nothing.
Please let me, I am willing to put type over the light.

Thanks
Bobbi Leblanc[/b]