You'll need to get some diagnostic data here to figure that out.
I would get a good temp & fan monitoring app to see what it tells you. I like Temperature Gauge Pro but others will work as well here.
Install it and take a few screen shots on what it shows (Just after you start and after a while) and post them here (click on the blue square edit ribbon icon to post).
You could also run Apple Hardware Test follow this Apple TN on how AHT to see if any error pop. give us the full error message if you do get one.