Monitor/tv Samsung T27A550 freezes and stop working

[vanessamartinez]

sorry for my bad english, im portuguese speaker...

i have this monitor-tv t27a550 model (brazil) and something very weird is happening.
when connecting it to a cable tv decoder (by hdmi or av)... the monitor just 'freezes'' - its touch screen menu stops working, as its remote control, and volume goes from 0 to 100, it starts to change the source and the channels by itself...
the only solution is to unplug it from the docoder.

both equipments are connected to separated wall plugs. they dont have the ground pin on the plug (brazilian problem....)
none of them are connected to third equipments.

samsung couldnt find a solution and gave us a new display-tv, same model, which has the same problem.
a person from the cable-tv staff came and switched the cables and decoder, but it didnt solve the problem.

we really dont know what to do anymore.
has anyone seen this problem before???