When a module turns ON but not OFF it's generally an indication that the thing you turned on is generating noise/sucking signal that prevents the module from hearing the X10 commands. It turns on because the thing is off and not generating the noise. That thing is typically an LED light - but could be anything that's plugged in. One solution might be to plug an X10 filter into the module and the lights into the filter. Another is to add a hash choke to the load after the module. See this article:
http://jvde.us/x10/x10_cfl.pdf Which phase coupler/repeater do you have? Anything can go bad but it's more likely just not doing enough to get the job done anymore. The ABSOLUTE BEST solution is to install an XTB-IIR to solve signal issues. In the past I've tried various coupler repeaters from X10, Leviton, Smarthome and ACT but none of those come close to the XTB-IIR. See:
http://jvde.us/xtb-iir.htmI'm not affiliated with JVDE but I'm one of Jeff's biggest fans - his products are awesome.
Even with the XTB-IIR certain loads may still require a filter or hash choke. When I upgraded a pool light to LED, I had the same issue - it turn on but not off. I added a hash choke ($1 cost) to the circuit and all is well.