zomboman

It seems that all of the modpacks (without optifine) have very low FPS. I know that this is because I have a computer that has trouble running minecraft, but there has to be a way to fix this. The thing is, when I start it up it goes to a good 30-40 FPS, but after 10 seconds it foes to 15-20 and then all the way down to 8-15 and stays that way. I try to ignore it, but it is really hard to play with each frame jumping around. Here is my question: Is there any way to install optifine or any way to fix this?