I would suspect that high voltage is frying the DP34's even though both of the previous posters state that the electrical systems into which the receivers are plugged into are normal.
The chassis of the receiver is grounded to the coax, the dp34, and to the neutral wire in the house electrical system.
If a condition known to electrical engineers as a floating ground exists, voltages as high as 135 volts AC or more can feed to the DP34. This can occur intermitantly and only when certain other appliances are turned on in the house; those especially connected to the circuits using the companion couductor of the full 220 VAC circuit.
If you are not a qualified experienced electrician, one needs to check the electrical system for such a condition as a floating ground.