I have this same problem. After some analysis, I discovered that I am releasing the fabric as I near the end of a seam. I guess I'm afraid that I'll sew my hand under the needle.
If that is the case for you: When you near the end of the seam, make sure your left hand is guiding the fabric on the left side of the needle and your right hand is below the presser foot as you end the seam. The left hand should keep the fabric straight while going under the needle even if you release the fabric with your right hand.
It takes a little time, but with practice you will maintain even feeding of the fabric to the end of the seam.