DMX Problems

HELP! Down at Melville Theatre, we seem to have a ghost in the machine. The lighting rig is quite simple - A Jands ESP II desk, and two 12 channel Jands GP12c dimmer racks. The problem is intermittent. When we go to black, and then try to bring lights up, one or the other of the racks doesn't respond. The DMX light on the rack goes out, and the STATUS light flashes (with a 0.1 second period, about). If we then cycle the 3 phase supply to the dimmers, everything springs back to life. The fault doesn't happen every time. If we don't go all the way to black, the problem doesn't seem to occur. When it happens, sitting by the dimmer, I hear it start to ramp up for half a second or so, then it crashes. What we have tried - - Rented a replacement desk, also an ESP II, with no result. - Rented a dimmer rack - The rental rack has not yet failed to respond, but still have issues with whichever of our Jands dimmers we plug in. - replaced our DMX cables. The Jands dimmers have built in terminating resistor, which we are making sure is switched on at the final rack. The racks are correctly addressed. All the power supplies involved originate from the same switchboard. Today, I'm going to run the desk powered from an isolation transformer, in case it is an earth loop issue in the DMX cable. If that doesn't do it, I'm going to rent another dimmer rack and see what happens. Any ideas? We have had this problem for a long time, though it seems to come and go.