I also recently had some similar problems.
I have three headlamps MKIV series and they have not been used much after purchase.
I recently found out that of the three Headlamp the H604d is the one that maintains performance the longest before going down.
H604c is the one with the worst behavior (drops fastest of all three)
H600Fw an intermediate behavior between the two.
I used thermometers, stopwatch and lux meter to test using multiple batteries and also using the same battery on multiple flashlights.
A test at about 21 C ° (indoors) showed this H604c problematic, but the repeated test outdoors with 14C ° and a lot of wind gave different (better) results.
The H604c is always the worst of the three examples but with the wind and the low temperature the times before the stepdown change a lot (for the better)
I tried again at home using also a hairdryer when I see the light go down (cold air used at minimum)
The Zebralights seem very sensitive to air circulation, just a few seconds of hair dryer (even kept away!)
A few seconds of air are enough to see the power increase immediately (visible even without lux meter)
I opened a case with Zebralight cs, the communications were difficult because there are no answers using the email while they are immediate by writing through the account on their site
They say that the PID oscillations are not visible but I see them well, both with the instrument and with the naked eye.
I took advantage of the event to analyze the behavior of my old H602w by doing tests at the 620 lumens level.
Here too it was interesting to see the behavior, constant light for many consecutive minutes and after around 10 minutes (homemade test - about 21 C ° - no fan) the power drops to about 480 lumens but just use the hair dryer at minimum (held away) for a few seconds to see the 620 lumens return immediately.
For H604c I hope to receive clarification from Zebralight, I find it absurd that three lights of the same series purchased a few months from each other have given different results.
This being the case, my H604c can only be used at high power at low temperatures.
I tried to follow the instructions and change the PID value but without noticing any change.
For the moment they told me to clean the contacts but I don't have Deoxit, I'm waiting for their answer to find out if I can use homemade alcohol or a specific WD 40 for electrical contacts.
The behavior of my Armytek is different, they keep the maximum power longer without going down but the new v4 versions have a differently calibrated thermal sensor and will be similar to the Zebra.
I heard from a dealer that there are many complaints about excessive heat but with my Armytek I never got burned :-D
However, I want to clarify that the Armytek Pro is bigger, heavier and also has cooling fins.
The old Wizard Pro V3 has a thermal sensor that intervenes at 60 or 65 C ° while the Zebralights never get so hot, with an infrared thermometer I have always detected lower temperatures (they change depending on the measured point) I do not seem to ever having seen more than 55 ° C