Gameboy Advance Slowing down

Hey guys, how are you?

I am facing this problem with a GBA I just got from eBay. It seems to work fine, but after 5 minutes playing the game just start to slow down a lot. I tested different games and the same happens. Actually, if I just turn the system on, the logo appears in slow motion. Does anyone have the same issue? The funny thing is if I leave it alone for a while it comes back to normal, but if I start playing, it slowdowns again.