I found this in the FAQ:

6.22) Oven heats on high setting regardless of power setting


Power levels in a microwave oven are controlled by cycling the
microwave
generator on and off with a variable duty cycle - kind of like slow
pulse
width modulation. For 'HIGH', it runs continuously; for low, it may
run
10% on and 90% off; other settings are in between.

When the oven always seems to be stuck at high power, it is likely to
be
due to one of two possible causes - a faulty relay or Triac, or
controller.
The relay or triac may have failed in the on state. This will probably
show up with ohmmeter tests (with the oven unplugged!) but not always.

Replacements should be readily available. If the problem is is the
controller, it will be more difficult to diagnose as schematics for the
controller are usually not readily available. However, it could be
something
simple like a bad connection or dirty connector.

*******

I was assuming that under normal mode (high?), the microwave cycles,
but apparently, it runs continuously. The problem I am having is that
I used to be able to heat a cup of water to near boil in about 3
minutes. Now when I heat 3 minutes, the water is boiling furiously.
Either the normal mode was doing some cycling and now it is not or the
magnetron is now putting out more energy (is this likely?).