2001 Isuzu Rodeo keeps stalling

We found a nice 2001 Rodeo for sale from a private owner and really liked it. So we took it to our local mechanic to have it checked before we offered to buy it. They found the rear brakes and fan clutch needed replacing. No big deals there. While they had it the “Check engine soon” light came on and they put the scanner on it and it showed “PO446 Evaporative Emission System Vent Control Circuit.” I was told that this wasn’t a serious problem and could take care of it later. They re-set the Check Engine light. So I bought the car from the owner. Now the Check Engine light is back on and the car occasionally stalls when coming to a stop, and today, it stalled while driving. It always re-starts easily, but not knowing when it’s going to die is a real problem. The car has about 86k on it and has the 3.2L V6 and 4WD. The car is back in the shop now and they are having a hard time figuring out what’s wrong. They don’t think the Vent issue mentioned before is the problem. Any ideas?