I have a 1991 Chevy Astro Van, 4.3 Vortec with over 700,000 miles. Just recently it started to stall when I have the a/c running and put it in reverse. It runs perfect going forward with a/c running. My longtime mechanic cannot figure it out. I have taken it to a couple of other mechanics with no luck. Any ideas other than, " don't back-up" ?