Couple of thoughts: You could turn the power down depending on the wattage of your micro there's a big difference in max power between 1,000 & 900 watt micros. The other possibility to explain your problem is that you may have a bad batch of plastic, it happens every now and then regardless of mfg. I'm assuming your micro is a rotating one not a stationary which makes a big difference as well. To narrow down the issue you could heat up 6 oz in a small pot and see what happens, turns yellow probably the plastic if it doesn't probably the micro.
yea I know pain in the rear.