On the side of the unit, are the "bumps" in the sled digging into the side of the unit? This was a problem with the Mk1 sled, but the mk2 sled has "sprung" bumps which have (as far as I'm aware) solved the problem. I suspect if your car doesn't let these flex outwards at all then you might be suffering from this problem though.

Where is the resistance - just on pulling out? Does it become easier to pull/push at any point (if so, how far out of the sled is the empeg at that point?)

Hugo