Doesn't sound like a frame rate issue from the OP.
Serkee, what are your machine's specs? Ram/Graphics card?
Is your computer a laptop with both integrated and discrete graphics? It could be an issue with the graphics drivers not recognizing Minecraft as a 3D application and switching graphisc processing power to it.
Advanced 3D graphics featuring NVIDIA® Quadro® FX 770M or Quadro® FX 1700M graphics engine with 512MB of dedicated graphics1 memory for demanding tasks such as, video editing, animation, and CAD manipulation
An estimated starting weight of only 5.92lbs2 (2.69kg)
Intel® Core™2 Duo, Core™2 Extreme, and Core™2 Quad Extreme Edition processors for number crunching performance
A long-life battery to maximize productivity
A crisp 15.4" high resolution displays including the 100% Adobe® Color Gamut RGB LED Display
A choice of 32bit and 64bit operating systems including Red Hat® Linux® designed to take advantage of a wide range of workstation applications
Wireless communication with Wi-Fi, Mobile Broadband3, Bluetooth® and HyperBand Diversity Antenna
Dell ControlPointTM to easily manage your wireless Internet connection
Comprehensive security and data protection features such as a robust software suite, multi-level user authentication, and secure e-mail and web access
I pretty much doubt the graphics or my laptop has anything to do with it. It just happens some times.