did you check for crossed pairs? DC continuity check will not show this problem. If you have not properly kept the pairs on the correct pins, you completley lose the noise canceling property of the twisted pair, in fact, you generate noise. While the noise may not be an issue on a real short run, it will be on longer runs, causing bad packets. The 4m cable that acts "slower" would be an indicaiton of this problem. Good luck
