Stalling on down hill off ramps after long travel

I have a 99 Bonneville Pontiac with around 120,000 miles on it. It's a V6. Three times now this problem happened. I'll be coming back from my parents (a 1.5-2 hr drive) and when I hit the off ramp to come into the city where I live, it dies. The radio is still on so it's not a complete power failure but the engine cuts away and I'm left rolling down the hill at best to get it off to the side of the road. Sometimes I can restart it fairly soon to get off the road but it doesn't last long. It needs to sit for a little bit before travelling back to my house or the local repair shop. It doesn't happen when I go to my parents, only when I'm entering my home town on the off ramps. It's the same distance and same roads. I don't get it. The off ramps here are all typically downhill and the ones at my parents could be either a downhill or an uphill, but I do use the up one more often. I don't understand and the repair shop said they have no clue and can't help.