Ahhh... I finally have the chance to post from a computer with a real browser. This site doesn't seem to like my phone's browser.
I'm sure your PID controller could be tuned better- even a 10 degree temperature swing seems way too much. Mine is only +/-1 degree and that's with rough numbers from memory since my controller's dynamic self tune feature really screwed things up after the initial auto tune.
You can try to tweak your settings yourself. Your manual recommends increasing the Integral time. Or you can run the auto tuning again. I know with my brand of controller, it was best to start it with the default P, I, and D control values, whatever they were. You might need to experiment a bit. Probably auto tune with something in the smoker. If not a hunk of meat, at least a brick or a covered pot of water.
Was the PV reading on the controller actually steady or did it follow the graphs from your Stoker? In other words, the Stoker indicates the smoker never really came up to the temp you set it at, but I find it hard to believe the controller was cutting out too early.
It's also interesting to note that with a steadier temperature, the heat within the smoker was more even. Rather, it's not that the back was consistently 15 degrees warmer than the front, it almost like it was one third of the temperature swing hotter. I'm sure it's not quite like that, but I've only got two data points to work with.... Okay, the other data point would be at room temperature, so a straight 15 degree delta would be pretty hard to achieve then.
Can you use your Stoker for measuring the food temperature? With some creative wiring you might be able to get the Stoker to put the smoker in a hold mode where the temp is reduced.
Finally, on your site you're worried about driving the smoker too hot- in industrial applications, there is a separate limit alarm box to warn when things get too hot. The PID controller can be set to trigger one of the alarm outputs if things get too hot. If you want, you can put a relay on the SSR control line to turn it off when the alarm is triggered, however, a friend told me that the failure mode of SSRs is to short closed, so turning it off probably wouldn't work. Instead, think about putting a relay on the high voltage side of the SSR. If the alarm triggers, the relay would hopefully cut power to the heater.