Well, maybe it's not quite as bad as rollercoaster but the temperature on my electric stove does indeed cycle on and off and on and off, which results in a slight drop and then increase in the temperature of the contents in the pot. I have measured variations where the swing is from 170F to 178F during the on/off cycles that happen with most (if not all) electric stoves.
Descriptions of heat diffusers seem to only say that they spread out the heat more evenly to prevent hot spots on the bottom of the pot. But hot spots aren't my problem.
Do you know of a material that is resistant to temperature change such that it holds on to heat and maintains it's hot temperature even during the off portion of the stove's cycle? I'd be happy if I can keep the temparature swing within a range of only two degrees like from 172F to 174F. What material can i put on top of my stove that won't cool off so fast during the off portion of the stove's cycle?