Mechanics!! 2000 Mercury Cougar V6 problem...?

Thanks for mentioning you drive in snow. I'd suspect bad wheel bearings. My uncle has a mustard yellow 1970 Ford F-250 truck, and it couldn't hold a straight line on dry pavement in the summer. I thought it was an alignment problem. He told me he had rolled it onto its side trying to avoid a deer, and some time later said that new bearings had completely fixed the problem. Apparently the rain and dirt had corroded the bearings. I think the way you test for bad bearings is to literally kick the tires and see how solid the wheels are, with either the wheels on or off the ground.