The Intel Core i5 processor will do that. It is designed to - to boost performance with acceleration. When the temperature gets high, YES, it is designed to drop down to prevent overheating.
If it gets too hot, it will shut down the tablet instantly and will not turn on until it cools down. You don't want it to do that.
Linux, by the way, does the same thing and has the same driver set (created by Intel) as what Windows and Apple's OS/X (UNIX) has as well.
Diablo 3 requires some serious resources and don't recommend that particular game on a tablet of any model except maybe the i7 model coming out in August. That one will be able to handle it.
Now, if it is doing that with regular programs, you can manually shut down or change your power management plan. You may be able to do that with Diablo 3 and give that a shot.