Problem with first two stoker cooks - too hot!

My first two stoker cooks have not turned out exactly as expected. First was a butt, second was a 9 pound brisket.

Weather: cool (around 35 degrees f), some wind.
Pit target: (225)
Pan: WSM, foil, no water.
Fuel: RO charcoal
Top vent: 1/3.
Lighting: Minion with about 10-12 coals, placed near blower.
Amount of fuel: mostly full ring.

Okay - it moved up to 225 quite nicely within about 15 minutes, where it stays for about an hour to hour and a half. Looks like little fan activity after it initialize stabilizes.
Then it sets course for 270. No fan. And stays there for a long time - as in hours, with nothing going on but the alarm! I adjust the top vent to let out just a tiny bit. At around hour 8 it starts to come down. When it gets back to target, it starts to struggle to keep the fire going, and temp just drops even with vent opened back to 1/3. Stokerlog screams "fire out" and temperature continues to plunge.
In both cases, I took the meat out, and foiled. There still was a reasonable amount of coals left, and getting a pile of air in removing the food get it new life.

What am I doing wrong?

Murray,

Not real sure, I never had that problem.
First of all are all your lower vents closed except for the one the fan is hooked to?
Are you mounting the fan the right side up? It should be mounted so the wires are at the top, that way the internal damper can close off when the fan is not running.
Are you just wrapping the pan in foil or are you also using rolled up foil balls in the bottom of pan with another layer of foil over the top, to form a layer of insulating air? This helps to keep the radiant heat under control.
I use the 14" clay pot base in the water pan wrapped in foil.
It sounds to me you may have air leakage around the door or possibly the body of wsm is a little out of round allowing too much air flow, if everything else is set up right.

I agree with Randy, make sure you have the fan right side up. With the top vent at 1/3, it shouldn't run up on you like that. Have you checked the calibration of your probes? You do have the pit temp probe set as fire, right?

Yes, I'm sure I had the fan right side up. (I checked that the little door would fall with gravity). And I had the probes the right way around. Clearly the fire probe went up, and the meat probe stayed put and climbed appropriately including plateau at 160.

I didn't have the foil balls, as I thought previously that it was just to help keep drippings from burning. My thinking it that radiant heat, if it was not under control, should only cause the meat to cook unevenly, not impact temperature as the heat has to go somewhere.

The thing is, at first, everything looks right with the fan going until target temperature, then backing off, then cycling on as it drops below, going off, ...

I'll double check my fan installation, but ..

Also, without stoker, I can keep the fire at 225, though it requires some regulation. (Hence the stoker, as I've got to have a life somedays

Seeing as your temperature kept going up even though the fan wasn't running, I'm guessing you have an air leak somewhere. The only air leak that the Stoker can cause is if that flapper door gets stuck open (although I've never had this happen).