I constructed that a sled using those plans a few months ago. I modified things slightly, and it works excellent (although took a few iterations to get it working well).
I'd recommend replacing your runner material with something harder. I started with hard maple. These worked ok, but not great. I recently upgraded to steel bar, available at the local BORG. This has made a huge difference in overall stiffness and ease of sliding the sled. One problem I had with the wooden runners was that they were very slightly warped, or curved, so if the sled was square at the front of the cut, it was slightly off by the end. This did cause some binding.
I'd also recommend attaching a piece of 2x4 or 2x6 protruding out the back. This is similar to the 'tunnel' someone else mentioned, but a lot simpler. The blade goes into the 2x4, and I always know where it will end up after a cut. I don't have to worry about visually checking where my hand is - I can do it by feel.
Try replacing the runners, and see if that helps. A working sled is well worth the time invested.
Chad