Check Engine Light On problems of the 2003 Mazda Protege 5

While driving on highway at about 60 mph for 40 minutes, the car began sputtering, jerking, making noises, losing acceleration and the check engine light came on. It initially felt like someone was taking their foot on and off the gas. We pulled off road and were towed to our garage. Mechanic found that the spark plug for cylinder 4 was smashed shut, he replaced it and told us to take it to the dealer since this was the 3rd time we had this problem. In Dec of 2009 the spark plug for cylinder 1 had same problem then again in Feb 2010 it was the plugs for 1 & 4. Each time we were driving at 55 mph or higher when car began to sputter then check engine light came on. Mazda is telling us nothing is in the engine, the compression is fine and they have no idea why spark plugs are getting smashed. The first time it happened our mechanic thought it was bad spark plug and replaced it free of charge, the second time we took it to the dealer who could find nothing. It is now at the dealer again and they are saying the only thing they could do is take the engine apart and look but have no guarantee of finding anything and it could cost $1500. They removed spark plug, checked for debris and tested the compression. They also used a magnet to search for foreign body in cylinder and found nothing.