Trackpad sometimes not tracking

I have a MacBook that I bought used and I was unhappy to find out that the trackpad doesn't work 100%. It often stops tracking when you move our finger across the center of the trackpad. It will often not be able to pick up the scroll gesture properly. Very annoying, I love macs excatly for their quality track pads. Any suggestions for how to fix the issue?

If I have to replace the hardware, I'd like to replace the part myself. I don't see a part listed for 15" trackpad, can you just use the trackpad for the 13"?