I have a 1996 XLT 4.0l. When I am on the highway taking a long drive, the A/C blowing and working great, all of a sudden, the air stops blowing out the vents and starts blowing out of the defrost. Still cold, but strange, after a few min. It will switch back to the vents.this havens every now and then, and mostly when I'm on the highway on a longer trip after about a hour or so of use. Has anyone had this issue ?