1998 Grand Prix with 130K.... car rund great most of the time but every so often when it rains or is expremely humid, it will stat sputtering when you press on the accelerator and if you press harder it gets worse until the cadalac converter gets heated up. Then after letting it sit for a while, it works fine again. Why is this happening? I have taken it to a couple different mechanics and while I think it might have something to do with the fuel filter/fuel pump or cadalac converter, I am told it isn't my cadalac converter! Help!!