Prime95 errors on non overclocked e6400!!!!!!!!!

I just installed my new core 2 system. All the specs are listed below. I just ran dual prime95 for the first time and the second core had an error after only 15 minutes. the system is not overclocked. it runs smoothly besides that. the only thing i noticed is that my temps seem high, but not too much. Core temp and intel thermal analysis tool both say the same thing under load, 64 degrees. But speedfan and easytune 5 both say 56. obviously thats higher then normal, but its not horrible. I have a new heatsink coming already with ac 5. But why would prime95 error already with these temps and no overclock? help! and it errored after only 15 minutes. should i rma?