While two repeaters in the same room should be good and it's suspicious that it's affecting more than one, I can't recommend replacing the antenna in these things enough. My problems with them disapeared after doing this. Even just opening it up and making sure the stock antenna is securely connected might be a good start (someone here said they noticed that too).
For future reference, you shouldn't need to delete the device from HE to re-pair, though if you've had it paired for a long time (more than the last couple firmware versions), you did yourself a favor by doing that at least once--now the child devices don't refer to the parent DNI and will continue to fully function if you ever reset and re-pair the device (as you've tried). Like most Zigbee devices, if you leave them in the hub DB, they'll fall back into place on a re-pair. (I don't think that explains any problems here.)
