Flame Boss 200 held steady + then spiked

You have air leakage somewhere. 

Thanks for posting your graph 'cuz that makes it much easier to offer some advice.  Notice that your fan never really ran.  Once the temp reached target at about 12:40, then it burped on a little here or there.  Shut down your top even more.  I have to completely close the Smokeware cap on one of my eggs to hold the temp at 225*.

The Flameboss and Guru controllers can only turn the fan on or off.  They turn the fan on when the temp is below target, and turn it off when at or above target.  Once the temp goes above target, the only thing that will bring it down is to close off the natural airflow ... close the top vent even to the extent that it's completely closed. 

If that still doesn't work, we can investigate closing off the bottom vent some by adding a little tape to throttle natural airflow at that end of the egg.  Shouldn't need to though.

Couple of questions: 1) Is your egg exposed to the wind?  2)  Is the wind strong today?

TheRainMan said:

... I moved my FB grill probe up to the BGE thermometer (in case I was getting a bad reading coming up from the side of the platesetter.

Never put water in the drip pan (always did w my Weber, but was thinking that it was unnecessary w BGE.

I have a XL

If you don't protect the BGE thermometer and the controller probe from direct radiation from the lump, then you can have problems midway through a cook.  On a low-n-slow, the fire doesn't always stay in the middle of the lump pit.  Sometimes, just enough to screw with you, it'll move forward so the dome thermometer "sees" the fire and, even though the average internal temp is the same, the thermometer starts registering a higher temp.

You had everything balanced for a "manual" cook from about 12:40 until 2:10, then something changed to increase the temp.  Figure out what changed at 2:10 and you'll have it solved.

Little Steven said it best ... water in the drip pan will screw up a cook.  You don't need the moisture in a ceramic egg.  Plus, as soon as the water evaporates, the temps will go up.

XL's have too much mass for the step function in temp that occurred at 2:10 ... something changed to increase the airflow.

If it has a blower and damper that looks just like what happens if the damper is open too much

@krames ... notice how your temp was 30 plus degrees above target and your fan wasn't running.  That's a sure sign that you don't have the top vent closed enough. The top vents need to be closed much more than you're accustom to with manual control.

TheRainMan is now cooking right along ... ask him what he ended up doing to achieve the correct airflow.

According to the man from Flameboss ,open top vent the thickness of a dime.I have been using an Auber control for a couple of years,anytime I had temp control problems it was my fault because top vent was not closed enough.Dime thickness is about 1/16 inch . I use Smokeware cap on my Egg.I have Generation 1 and 2 Smokewares and both need to be closed down to 1/16.I also use aluminum tape around blower motor to seal any air from entering .Auber blower motor is same as Flameboss but Auber blower has an automatic damper that swings shut when motor is off .Maybe you are getting air from wind,heard that could be a Flameboss problem ,people have fabricated motor covers to stop wind from affecting things . When mounting motor to Egg see if you can install motor so that opening faces down ,that is how Auber wants their fan installed,might stop wind from affecting things.I am one of those people that use a pan for many of my cooks,beer ,apple juice etc. No problem with spikes on low and slows.Hope this helps .

@Jeepster47 Thanks for the tip! I am pretty sure I had my top vent only opened a crack, but I will make sure next time.

I am also lacking a gasket on the top of the egg and I think the gasket on the lid is going. It might be time for a new one.