Ignition coil problem?

I recently purchased a 99 tracker with a 1.6 engine. It starts and runs fine-EXCEPT after running about 30 minutes-it chugs and stops. I wait about five minutes and it starts again. I had Firestone run a computer check and it stated it needed new ignition coils (Expensive!) Unfortunately this did not fix the problem ANY IDEAS AS TO THE CAUSE??? Engine does not overheat.

Does it make any colors of exhaust before it stalls? black, blue, white? Does it just stall and die quietly?

There is not smoke, just dies. If the vehicle is running is spurts along and then stops. It feels like the car is either running out of gas or no spark.