One thing I've also noticed is that some brands are really sensitive to the climate. For example, I have some Pacer Z Poxy (30 minute) that appears to be sensitive to humidity. Every other batch cures really rubbery and to be sure I've taken time to dispense it accurately and mix it thoroughly. I finally tossed the stuff out. I'm not sure what the problem was but I'm guessing the moisture in the air had a strong effect on it.

I'm using Devcon now and not having any problems. The Z-poxy worked fine in less humid climate (Germany). Maybe it was just old, but I had it less than a year. I've had other brands that lasted much longer than that.