Do you multi-task your computer while playing? Using programs such as Skype or browsing through the web and also playing minecraft can eat up your processor and your RAM, mostly if it's a modpack.
Go to your task manager, on the performance tab and see if it's the the CPU or the RAM.
If it's your CPU usage going up dramatically, it's probably a program you've installed that is running, without you noticing. I've been using
Game Booster
- It stops programs and services that you do not need when you are playing games, and you can restore the program once you are done with your gaming session.
If you still have issues, you will need to find a way to get more airflow into your computer, find a way to cool down your processor, or upgrade your rig in general.
You won't need a high-end video card, since it's minecraft afterall. Somewhere inbetween the Radeon 5000 series and the mid GTX 400+ will be appropriate enough for minecraft and other games that don't take much graphics and textures.
Also, make sure you had gone through the settings on the FTB program so set how much RAM you give to the program.
I've used optifine 1.4.6 HD_D5 and i haven't had texture problems. Maybe using optifine and a texture pack can lower some frames down.
Goodluck on your issues.