Do you ever notice that your left thumb is touching the top of the trackpad when you are using it?
I have noticed that the large size of the macbook pro trackpads causes me to accidentally rest my left thumb on it, causing odd trackpad behaviors similar to what you are discribing.
This happens for me, mostly because I only use Windows for FPS gaming, which has a different standard hand position than typing. I think this is what causes me to have more troubles in MacOSX than Windows. Since I realized this I have trained myself to keep my left hand clear of the trackpad unless I need it.
Hope that helps.