Below is a link to a post I made a about 1.5 yrs ago where we were discussing microwaves and how long they continue to cook (and raise temperature) AFTER you pull the cup of plastic out of the microwave.
Also discussed is how your high wattage microwave could make a
big difference in heating (and burning ...which is what the yellowing is) of your plastic.
I have very little issues with yellowing/burning plastic...but maybe it's because I use a low wattage microwave now.
I also heat in smaller increments...4 oz for 2 minutes, then in 20-30 second intervals to get it to temperature. I never "reheat" more than 1 minute at a time. I've had plastic go from too thick to inject (with clumps from the sprue, runner and left over injector plastic) to burning in less than a 2 minute heat cycle.
Thinking back, I did have a high wattage microwave and I did have issues with plastic yellowing (scorching) back them ...and not just CCM plastic which was "new" back then. I remember setting my microwave to 50% power to keep from burning the plastic (and obviously learning to go to shorter heat cycle times).
The other "Must" is a turntable in your microwave to avoid spot heating which is a big issue with cooking food in a microwave. There is no doubt that scorching starts and the bottom center of the Pyrex cup.
I wonder how much the cups we are using to heat our plastic is contributing to the over heating issue??? I used silicon cups (Jason recommends them) for a long time and the do heat much more evenly than Pyrex. I just got nervous about how floppy they were.
http://www.caneycreekmolds.net/index.php?topic=667.msg3362#msg3362 From that post:
Also, remember that anything that is microwaved may continue to "cook" for a period of time (the molecules stay excited and vibrating)!!! Don't think so? Heat you plastic to 350 and stir with the digital food thermometer (the long stainless steel probe is perfect for that) and watch how high you temperature goes!!! I'd bet if you are heating to 375...the plastic is going over 400 degrees within the first 30 seconds out of the microwave.
From:
http://www.fsis.usda.gov/factsheets/microwave_ovens_and_food_safety/index.asp#14Microwaves cause water, fat, and sugar molecules to vibrate 2.5 million times per second, producing heat. After the oven is off or food is removed from the oven, the molecules continue to generate heat as they come to a standstill. This additional cooking after microwaving stops is called "carryover cooking time," "resting time," or "standing time." It occurs for a longer time in dense foods such as a whole turkey or beef roast than in less-dense foods like breads, small vegetables and fruits. During this time, the temperature of a food can increase several degrees. For that reason, directions may advise to let a food "rest" for a few minutes after turning off the oven or removing food from the oven.
I set my microwave down to medium at the end of the heating cycle...or I often get scorching. This is probably more important if you have a high wattage microwave ...which will cook faster than a lower wattage unit.
(read the "Time-to-Boil Test" in the above mentioned article).