spam chucker

I usually use straight devcon, no thinners, and have great results. Today was a little cold, so i heated it up in the microwave in a plastic cup for 7 seconds. While mixing for about 20 seconds it started to yellow suddenly, then set up in about 10 seconds. Within a minute I had a yellowish hard blob and a half melted cup. How can i make the epoxy flow better without getting the same results?

this inspires the tackle hos to take it to another level. Thanks a bunch!