170 is the value used to sync the pixelnet data. All data values of 170 are converted to 171 in the etherdongle when generating the pixelnet data stream. I'm not sure if this is true in DMX mode, but it probably is...
If the dimming is not smooth and always acts up at a certain value, look to the device... You were purposely vague as to what the device is... The data stream is pretty much an all or nothing thing if the line is properly terminated (there needs to be a 120 ohm resistor across the data pair at the last device. DMX should be daisy chained from one device to another. Etc.)
Are you using a hub or going strait to the device.
LEDs plugged into AC dimmers will not come on until the threshold value is reached. This is one of the advantages to using a lynx express with the proper light curve installed, it will remap the DMX values to the led string's active range. Led strings on AC dimmers also tend to top out in brightness before you get to 255.
RM