not good to here your first nitro car is a pain in the butt hpi is a safe make to go with aswell, so you could just have one of those bad eggs so to speak, next time a plug goes or you take it out to check it have a look in the top of the engine to make sure you dont have an element from a previous blown plug bouncing around inside. I have had that before and just kept blowing plugs 1 after the other.
HPI have one of the best warranty's going in the rc world so if its becoming to much of a pain contact the people you got it from and they may be able to get replacement parts like engine etc from HPI. This is not guaranteed because normally engines are not covered once fuel has passed through them but its got to be worth a try.