Two Legged Walk Cycle - Body Translation Issue

I’m new to animation:

I have a basic model with two legs, reverse facing knees (like a raptor). I’m using a total of 12 key frames. Both legs are moving correctly. I moved the model along the path (straight line) to key frame 6 (the middle mark on the timeline) and there is no foot slippage. However, when I try to move the model along the same path for the last 6 key frames (last mark on the timeline) I get huge foot slippage right from the first key frame. I can supply more pics if needed…