Where to begin.
I had a case where(OK, 2 cases) where MyDMX took a poop on me at a show. The first time, I had no time, went into fail-over mode and used my DMX Operator. The second show(same band, same venue, same computer, same everything) and I quit MyDMX, re-plugged the dongle, re-started MyDMX, and I was happy.
First, I'd check the physical(cabling) and ensure all are DMX cables(best results) and yu have total end to end connectivity. Stringing them all together and then running a cable tester works great for this quick test.
Check your addresses as well.
Is this a fresh MyDMX install? As in perhaps from the web site, dated June 5, 2008(when you see it load).
If time permits, set up a new universe? Sometimes it's best to start fresh.
I can't even relate to these problems. When MyDMX was crapping out on me, it would go into mega super slow-mo mode, IN addition to NOT sending data. I'd typically test this by running MyDMX with the dongle plugged in, and hitting "W" for my "all white" scene. If it is complete within 2 seconds(regardless if lights are attached), then I know MyDMX is working fine. When I have the problem, it can take up to 5 minutes for it to do the same thing on the screen.
You are right, a fader should still send data on the channel with the values of 0-255. The advantage of having a "fixture" installed or associated with those channels is that you get the benefit of the profile to help you make changes more intelligently on the fly.