My 2008 AIR demands to be shut down about 5 minutes into any download involving music or video wassup with this ?

My recently acquired 13" original 2008 AIR has a nasty quirk. It cant stream music or video for more than 5 minutes before demanding to be manually shut down and restarted. I swapped the hard drive (SDD) for the earlier ATA one....no diff. I suspect a bad fan, but does anybody else have experience with this syndrome?Thanks.