Lykle,
I have a theory (but it's unproven) that cartridge heaters "age" when you start pushing them to higher temperatures. I have one that used to do some testing on heat sink compound and which I pushed up to 400 deg C hot end temperature (yes I know it was extreme but had to be done in this case). Ever since then, that cartridge behaves completely different to any others (I have 10 of the things laying around). It still works but needs completely different PID settings to any others (and takes takes longer to reach the desired temperature). Maybe you had something similar happen and that was what you were seeing from one day to the next? Just a thought.
Ian
A resistance measurement between a good cartridge and an "aged" cartridge would go a long way to prove your theory. Short of some super thermal insulator forming inside the cartridge if the resistance stays the same it should work more or less the same, air gaps aside. Funny I found E3D's 30w heater to be 30 watts. I found some no name heaters I got off amazon which look good to instead of 40w be around 47w. Talk about too much heat.
Jeff