HD Graphics 4000 drop fps

I currently have an i5-3570k on a Z77 Gigabyte MB.

I have the gpu frequency to the top (3200) and the CPU running at 42 clock ratio

When I start a game its FPS are between 55-62 but suddenly as the game goes by it drops until 30-36 and won't go higher no matter what.

I was wondering if this is a problem or is normal.

I'm running under Wine on Linux.