Skinning problem

[JoeBun]

Hi, I have a polygon character which I've rigged and skinned (in Maya 5). Everything's virtually perfect except for a few small problems. One of which is this one: when I move the character's legs, at specific poses, the vertexes go a little 'crazy.' The front vertexes shoot back through the leg and the back ones shoot forward. I've included pictures of this problem. The first image is the leg with the foot up, the second is the foot move up by a few minute degrees. As you can see the vertexes from the front shoot out the back of the leg. Has anyone else had this problem before?

http://www.chi-yun.com/temp/foot1.jpg
normal...

http://www.chi-yun.com/temp/foot2.jpg
crazy!!

And this is not the only situation, there are a few other poses that this will happen with. Anyway, I'd appreciate any help.
Thanks!