I am getting frame drops in single player, I'm playing on ultra settings, 1920 x 1080 with dx11 and high res texture packs and v sync off

My frames can go up into the 90's in some area's but it seems when I'm on a part where there is a lot of action my frames will drop into the 30's now and again and sometimes a little bit lower (maybe 29), is this normal?

my system is:

AMD Bulldozer FX 6100 @3.3Ghz
Msi Gtx 670 OC 2gb
8gb DDR3 1600Mhz
Asus asus m5a78l-m lx Motherboard
700Gb WD HDD
Windows 7 Ultimate 64bit

really need some help