It all comes down to a couple of things.
There is clearly some sort of problem with the cooling in your laptop. You haven't done anything to the components of the laptop, so it would be safe to assume that something is failing, more than likely the
CPU fan.
Now....depending on your confidence level with working around the insides of your laptop, you have at least 2 choices;
1) You call Sony and describe the fault and get them to send you a replacement
CPU fan/hestsink assembly (with instructions) that you fit...and feel incredibly pleased with yourself for doing so.
2) You get a local technician to do the same and pay more for the priviledge.