My UDS is set up just like yours I cook the same temp 300-325 No foil No injections. Your problem is the thermometer STOP using it. The only temp that you need concern yourself with is the pit temp. Butts come with a built-in indicator to let you know when it has achieved Nirvana the "Blade Bone". At 300-325 butts average 50 min lb I don't do the metric system so I'll let you do the conversion on that but is is a good indicator as to when Start checking if it ( the Blade Bone) is loose. I the meat is tight to pull It needs to cook longer.
BBQ RULES FOR SUCCESS
YOU CAN NOT COOK GREAT BBQ ON A CONSISTENT BASIS BY COOKING TO AN INTERNAL TEMPERATURE OR BY TIME ( XXX MIN PER LB) YOU MUST COOK BY FEEL! For Brisket it must pass the poke test(probe like soft butter in the thickest part of the Flat) Ribs pass the Bend Test, Pork Butts when the bone wiggles loose. These are the only reliable methods to ensure that your cook will be a success. There is one exception to these rules and that is Poultry which must achieve and internal temp of 170 deg in the thickest part of the thigh and 165 in the breast.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC