1999 Dodge Durango Durango keeps stalling

Our 1999 Durango has been stalling out frequently after just a short drive. It can stall while driving, but the vast majority of the time its when it has stopped for a moment. The warmer it is outside the more quickly it gets to this stage. We have brought it to the mechanic at least 5 times and left it there for long periods of time. They changed a fan, did a full tune up, adjusted the idle and said they cant figure out what it is. In a nutshell the car stalls after being on for only a short period and continues to do so, until you finally get it home. It seems to get worse in the heat. I noticed the last time I was driving it, when I got it home, it was smoking from underneath the hood a lot. I have dumped more than I would have wanted into this car at this point, although it has low mileage and in otherwise excellent shape. It is due for an inspection this month and wondering if its worth fixing, is it something simple, get it to the pt where it will pass inspection or just move on. Do you have any idea what the issue could be and if we should just say move on after 11 yrs with the car.