Whirlpool microwave is rated at 1000 watts but performance is closer to a 700 watt microwave. Dealer claims that a magnetron either works or it doesn't work and that they don't just get weak.
My question is if magnetrons just die or can they lose power gradually until they are dead?

Here’s a quick & easy way to find out what’s taking place in there:

Fill a Microwave-safe container with 1 carefully measured liter of water, preferably at room temperature, about 70F, and measure its temperature (Fahrenheit) as accurately as possible. Write that temp. down, and place the container in the MW. Set the timer for 2:03, and hit start. (If yours is a 'mechanical timer' type, with just a dial, use a stopwatch - this must be accurately timed.)

Carefully measure the ‘end’ temperature and multiply the difference by 19.4. The result is the approximate energy gain in watts. (I know you’re wondering about those 3 seconds - it takes about that long for the magnetron tube’s filament to heat and start to ‘fire’, and we want exactly 2 minutes of heating)

Keep in mind that it’s normal for a microwave to produce less energy as it ages (hmmm… sounds familiar somehow!), but your results should be within about 50-75 watts of the rating.

This microwave is 3 months old. The dealer is giving me the round-a-round about the wattage strength. He claims that a magnetron either works or it doesn't. I want to know from someone who knows about magnetrons if what he said is true or a cop-out. Could there be a weak magnetron in this unit? Or can a 1000 magnetron operate at less than 1000 watts?