Why does my fs go from 100+ fps to 30 in minecraft?

Okay, so i'm playing a modded minecraft pack called resonant rise. When i record with dxtory (no frame limiters set) i get 30 fps… But when not recording in the same world with teh same settings, i get 70-80-100+ fps (the only time it goes down is when i fill a seared tank with lava, and even that gets 30 fps). However, when i record it never seems to go above 50-60, and most of the time it's down at 28-30. Playing at 1280x720 windowed, fancy graphics off and render distance on far. Fps is set to max and open gl is off. Using an EVGA GT 640 2gb, overclocked by 300 MZ and an amd A6 3650 cpu, with 8 gb of ddr3 ram (4 of it dedicated to mine craft). I'm recording to a separate HDD than my game and operating system. If anyone have an answer to this i would be greatly appreciative.

Added (1). i'm using dxtory, and even if i was using fraps it shouldn't be dropping my fps by THAT much - that's a 70 frame difference D

None of the system stuff matter, the default recording Frames Per Second is 29.97 so the recorder will automatically record at that, you just need to change the settings in the screen recorder to do anything about that.