Is the DMX cable terminated after the last device?
Are you using true DMX cables with twisted pairs and good shielding?
If i may ask a question, What type of light is flashing? Does this occur at any intensity or just when you operate the light at full intensity? Is the light DMX controlled or plugged into a dimmer pack or panel? I know some dimmer packs and other devices will flash the outputs if the AC line voltage drops below a certain threshold. When this issue occurs look at the light fixture or dimmer pack's display and see if it is also flashing. If yes the line voltage may have dropped either due to too many lights on one circuit or faulty AC wiring. If you can, measure the line voltage to the light with a volt meter while the issue is occuring to confirm the fixture or dimmer is getting adequate AC line voltage. Hope this helps.
I just missed Jeremy's last statement. I have experienced this as well and have two solutions for you. First is as i mentioned earlier make sure the fixtures have adequate line voltage to them. Second if you haven't already done so, use a DMX distributor to evenly distribute the DMX signal to groups of fixtures as opposed of daisy chaining all the fixtures together.