To control heat production or rather destruction of a CPU (yes, yours has a APU) the speed pulls back as the temp goes up.
Since it worked before the usual causes are dust buildup (keep reading), some heatsink compound dried and cracked or other.
Tell me if you kept up with the 1st of the month canned air routine as well as did you place it on a laptop cooling pad.
So I've noticed that once I go into games that my processor actually slows down, and my performance tanks because of it
I usually run at 2.65 GHz (.5 GHz overclock), and when I enter any steam game, my processor stays at 2.65GHz, then goes down to 1.74GHz or 1.47GHz after 20 seconds or so
If I exit the game, my processor stays at 1.47 GHz
I used to be able to play at 2.65GHz, but one day it just stopped working
HP Pavilion 15 Notebook PC
Processor- AMD A 10-5745 with Radeon(tm) HD Graphics (4CPU), ~2.1 GHz
Memory- 8192MB RAM
Graphics- AMD Radeon(TM) HD 8610G
Possible things that might affect it
Asus GPU Tweak
Bitdefender (though ive screwed with profiles, and I don't think "game" setting affects this)