I have a 1991 Chevy Astro van with 138,000 miles. It has been running fine. In the last week I have experienced two instances when after driving several miles and stopping for gas or shopping upon restarting and driving just a short distance the vehicle stalled by sputtering and then after a few restarts and getting it to catch its breath it ran fine again. Any ideas? The first time was just after topping off the tank with a few gallons of gas.