If it works at normal speeds only while a 220v appliance like an electric dryer or heater is operating, then the units are plugged into separate 110v legs (generally residential panels are supplied with two out-of-phase 110v which combine into one 220v). They should be moved so that both are on the same one, or something called a "phase coupler" can be installed to allow signals to cross over.
If you think about it, it's kind of mind-boggling that they can sometimes work at all on different split-phase legs because the signal would have to travel through a running 110v appliance, through ground and then another running 110v appliance on the other leg. Or go through the neighborhood to the pole-mounted transformer, in which case signals from the neighbors' Powerline adapters or million-amp arc welder could interfere.