Welcome to the forum, Implied and Zmurder and thanks for choosing us.
Zmurder, a couple of things come to mind.
A bit on Voltage etc.
Resistance is set, therefor Voltage controls heat output with all other factors held steady... Higher Volts = Higher Heat, Lower Volts = Lower Heat. The OEM power supply (Jameco #10081) is an Unregulated Power Supply. ie. Output Voltage is determined by Input Voltage. An example ... if your mains are running at 120V and your output is 12V then, when the mains are at 115V your transformer will output 11.5 V. This is a simple induction transformer (unregulated).
The Wikipedia article
http://en.wikipedia.org/wiki/Electromagnetic_induction on Electromagnetic Induction may give you a better understanding of this type of transformer, but the little picture may do as much...
Put 100 coils on one side, 10 coils on the other, as an example... 10:1 ratio 120:12 115:11.5
So resistance (PD) is fixed and the induction ratio (transformer) is fixed, leaving Line Voltage (mains) as a variable. It is possible that your line Voltage drops, either Grid wide or locally (in home) during the day.
Not so likely, but possible.
( Just to finish the lesson on transformers... A Regulated Power Supply has a fixed output. ie. Input Voltage might be 100V - 240V but output will be a steady X Volts . . . )
There are other possible factors. Since you perceive a temp drop, by feel, we assume there is an actual temp drop. Of course lighting can make a huge difference in the visual perception of 'clouds'...
Room temps are first suspect. Does the room temp change? Another related factor that can make a PD cooler are drafts. Using a fan during the day to stay cool? It will cool the PD too, even a ceiling fan can make a difference. Not saying you can't use a fan around a PD, just saying a drafty room will lower temps a bit.
We just learned a new factor this week... We were using Pammy's Sapele unit for travel, it runs cooler because of the denser wood, great for Pammy's asthma... and under normal circumstances just fine for me. But at the motel I noticed a considerable drop in temps...
we were setting the unit on a cool tile floor. The cool tile was sucking some of the heat right out the bottom of the unit. Moved the unit onto a room temp wooden coffee table and soon temps returned to normal.
Grinding your tobacco daily? How long it is sitting after grinding may be another factor... Moisture content affects vapor production...
There are other factors to be considered, I'm sure. Hope that points you in the right direction.
Edit: composing while you wrote that. Thanks, Prog, it was nice. I have never been to the South Rim, was sure there would be a lot of tourists and there were, but mostly in one or two spots. We saw the North Rim in October of 2001, less folks on that side. But either side is worth the visit ...
Sure Prog, there have been a few warranty fixes. But only one or two un-explained resistor failures. A few pieces of wood have mis-behaved (in all species), as wood will do sometimes... With double clips and a clip seating tool, loose heat exchanger issues are rare (unless neglected). A bit of re-enforcing heat shrink tubing on the transformer power plug stress relief (PD Land installed) helps prolong the life of the weakest point in the system...
Little things to improve, nothing radical. I
don't read the other vape threads these days, maybe they will come up with an instant heating, portable, variable temp vape, that requires no power and fit's in your palm for only $5.95, but wait, if you call right now...
I won't know, I'm gonna keep doing what I do...