When my hot end didn't heat up, discovered that the power resistor was a short. Replaced it today and the new one, which I measured at 5.8 Ohms before installing it, shorted out. Does anyone know what could be causing this?

If I disconnect the resistor leads then turn on the RAMPS 1.4, I measure 12 V across the hot end terminal. Printer is Prusa Mendel i2 with a J-Head hot end.

I would really like to get this up and running this weekend, as we are supposed to demo it on Monday :-(

Two resistors in a row? Into a short? Resistors usually go open circuit when they fail.
I think something else is shorting the resistor. How have you installed it? have you got insulation over the legs? If the legs are bare and you are bending them around the hot-end, then if they both contact the hot-end, the hot-end will be the short.
Another place that a short circuit might exist across the resistor is where you have connected it to cables. Once you have crimped (or soldered, or whatever) the resistor to the cables, you then have to make sure the two crimps don't touch. Some heat shrink or something similar has to go over each crimp. Also check at the terminals on the ramps board. It's very easy for a strand or two on one of the wires to bridge across to the other terminal and short out the supply.

Have you remeasured the resistance of the first resistor now that you have removed it? Did it still measure short?

A short across the 12V will cause other issues too. I saw in another post you have trouble with your steppers working. Fix this problem and the stepper motor problem might go away too.

You were right - the resistor was shorted to the nozzle. I insulated the leads, but there must be a flaw in the insulation. I'll take it apart shortly. And yes, the first resistor, now uninstalled, is fine.

Thanks!