Ok, if you're using Unity I recommend the Unity Asset pack instead of the FBX files as the pack was built for Unity and hopefully that should fix the jittering. On a side note, sometimes FBX Animations in general takes some tinkering in Unity to get them to act right.
If that's the case you probably have to check the settings of your models as that can effect the animations, if that doesn't work you might have to do some searching online for a fix, because usually ticking the Foot IK box fixes these types of issues.
