Doesn't sound you like you are not overcooking the meat, sounds like you are undercooking it.
Typically at 225-250 pork butts will take about 1.5 per pound to get to the point where you can do the bone wiggle test. (wiggle the bone in the pork butt, if it starts releasing the meat is done)
For brisket, I am not 100% sure what the timing is per pound to start probing. I will normally start to probe the meat at about 195ish internal temperature. And then do it every 30mins or so after that till the brisket probes like a knife in warm butter.
The thing with beef and pork, you can't bbq by time or by internal temperature. You use time and temperature as guidelines. You have to go by the feel or the way each piece of meat acts to test for doneness. Brisket - Probes like butter, Pork butt - bone wiggle test, Ribs - bend or toothpick test.
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