Have twin evinrude 225's 2003 model. They start & run fine for about 20 minutes. Then one will shut down. Then the other will shut down. Cannot restart for 20 minutes or so. Motors will not crank. No lights or alarm. Just quit and are completely dead. After the "waiting period" they will start right up and run for a while longer. It seems the slower you go the longer they wii run until shutting down. Mechanic hooked up the diagnostic computer to both motors and found no problems. Please advise. Thanks