XLR cable is at 1000-600 ohms, while DMX cable is at 120 ohms. 120 ohms is loaded for data communication applications.
While XLR and DMX may look identical, they aren't. Used DMX cabling for lighting and XLR for microphones.
I don't care how "high a quality" the brand is, if it's the wrong cable for the job, quality won't matter. But, if you're sticking with a good brand, then that's good, as long as you're using the right cable for the right job.
You have two choices. You can build your own cables or buy premade. That's really whatever works best for your application. ADJ's AccuCable comes in many lengths, including 3,5 10 feet and longer(25 is my favorite length in general, but it's not good with the lights are all next to each other).
Also, make yourself a few DMX terminators unless you can buy them. I bet you can buy them for a decent price if you can find them, but 5 minutes with a soldering iron and a 120-ohm 1/4" resistor should also do the trick just as well.
Between running proper cabling and a DMX terminator, that should help. BUT, as has been pointed out to me, MyDMX dongle dones't put out a very "strong" signal, so it's recommended to get a splitter/repeater such as the OptoBranch/4, which I also have in my rig anyways for convenience.
All this lighting stuff for a sound guy such as myself....
Also, because DMX and XLR look identical, don't rely on the fact that your crew SHOULD read what is on the cable, because some cable doesn't tell you what it is. DO segregate your DMX cabling in order to properly label it. In my case, I have put a wrap of white tape on both connectors of the ends on my DMX cabling. "White" because it rhymes with "light". Hey, works for me. Otherwise, it's too easy to confuse things.