When vehicle is cranked cold and driven approx. 10

When vehicle is cranked cold and driven approx. 10 to 12 miles, engine will shut down and rpm's will drop to nothing. This last only for a second or 2 and if you keep your foot on the accelator engine will reengage and rpm's will return to normal. This will continue to occur over the next 5 or 6 miles and the engine will then smooth out and it will not reoccur. The engine has completely gone dead on several occasions usually as you are coming to a stop. On one occasion recently however the engine lost all power at 60mph including the power steering. This problem does not seem to manifest itself in short stop and go trips. Its as if the engine has to reach a certain temperature for this to occur. The dealership has been unable to duplicate the problem.