I just looked up the datasheet for that resistor.http://www.ohmite.com/catalog/pdf/89_series.pdf
I observed this interesting tidbit therein:
"Derating: Linearly from 100% @ +25°C to 0% @ +275°C."
If we run at 225 Deg C, then we have to derate the resistor to 20% or
0.2 * 10W = 2W
We're running 12V through a 5 ohm resistor.
12V / 5ohm = 2.4A
2.4A * 12V = 28.8 W
We're dissipating 28.8 W from what is effectively a 2W resistor.
I'm not surprised that the resistors are dying.