02 Windstar stalls in rev

We have a 2002 Windstar mini van as a company work van that I drive alot. I have a weird problem... I was in an uphill parking spot, and as soon as I put the van in reverse and got off the brake it stalled (auto). This happened 4 more times before I said screw it and let the van coast down and put it into drive (worked no prob)...It happens infrequently now but is a pain...any idea what the problem could be?