I have used the X-10 Powermid a couple of years ago (ST539 – RE549). It did work fine for a couple of weeks and then developed the same problem . I have found that the problem douse not starts immediately after power on but after about an hour. I have removed the top of the pyramid and the problem was solved. The transformer inside this transmitter is heating up the unit until it fails. Try removing the top (4 screws in the bottom of the unit under the rubber) and operate it like this for a wile to see if this solves your problem, drilling air vent holes might solve your problem.