Well,let's try a couple more thoughts.
Like Dave says,a nice big packer would be a good thing.
If you can't find one,and are determined to cook brisket,we'll try to help.
Lowering the internal temp to 180�,probably won't work.
Some folks have had success,as Smokin' says,laying fatty bacon/pork fat/beef fat on the top and cooking.
I'm too lazy to do all that.
I like to cook a pork butt over it,though.
Now,a flat might come tender at 185�,or at 210�.
A remote probe indicates when you should open the door and check it, with something else.
Yes,you do lose about 25 mins,each time you open the door and let all the heat out.
That means you need to figure a longer cooking time.
Are you running an independent remote probe through a ball of foil,so you know what temp is going on where the flat is setting?
225� is a good easy reference point.
Like Smokin'asked,have you verified the accuracy of your probe-that will go into the meat?
Some folks like to take a warm mixture of beef broth,a little worcestershire,a little margarine,a little apple juice, and just kinda sling it on the flat,so you don't wash off the bark.
They try that every couple of hours.
When your remote reads 185�,pull it out and see if it will slide easily through the flat,topside to bottom side.
If not,you probably need to cook it longer.
Try again in 5 �increments.
When it feels done,lay out a couple sheets of heavy duty foil and place the flat in the center.
Pour about 1/3 cup of your mop-or however much doesn't run all over the place- over the flat and seal tightly.
Wrap in a big towel and let it rest about 3 hours in a dry cooler.
Next set a table for yourself,and maybe someone else.
If they would appreciate fine food.
Set out three cold Shiner Bocks-per diner,or a good German influenced beer from Wisconson.
A slice of Texas toast,some sliced pickled jalapenos,a little onion,and a bowl of potato salad.
Put on Guy Clarke's"Texas Cookin",or a little SRV.
When ready,slice across the grain.
If it is a little tough,slice thinner.
If it wants to break up ,slice a little thicker.
When you finish all that,let us know if we got lucky.
Now,I'll go-'fore someone accuses me of makin' a Smokin' post.