HelpWeepcraft RAM problem

I allocated 5 G's of RAM Into WeepCraft Client, and when I launch, it only allows me to use 12-18%, then jumps back to 1%. This is causing FPS problems for me, can anybody help?
EDIT:
I'm guessing it has something to do with this

How can I get more allocated memory?

How exactly are you allocating the ram? Please post a screenshot.

Well.. Its not going to use all of 5KMB of RAM now is it.. v.v That would be insanely 'un-resourceful'.
it uses as much memory as it requires at that point. Your low fps rate is coming not from the memory allocation but from.. few factors.. Maybe a new computer would do much better? Maybe close all the unwanted processes and programs? Maybe take care of your computer, update it, check it, de-fragment it?

Google it, there are plenty ways to allocate RAM.

Try the task manager, thats the simplest

RAM is not all that's needed to get good performance in Minecraft. CPU and GPU play a bigger role.

To put it simply, think of RAM as cake. If you were hungry enough to eat 1 and a half cakes, but was only given 1 cake, you'd eat all that cake and want more. However, if you only wanted to eat 1 and a half cakes, but were given 1,000 cakes, you'd only eat 1 and a half cakes, not 1,000. This relates to your problem in the way that your client only wants to use a certain amount at that moment, however if it ever needs more, it's right there.

Hopefully this helped you.

You know.. Just Buy a new pc, you will get much better performance/ram