Bama,get a decent thermometer and try again. I feel you trusted the stock therm. and way overcooked it,Beef is sometime stringy,depending on it's diet, and Pit was right; I know you are trying to get the best, but you need to know the Farmer too!
The therm. thingy is a must,talk to Creosote(Todd) about therms.,he'll do you right
Search the Wiki for temps. and times for your cook.A Chuck should have turned out tender for you,as I stated, you must have a temp. problem
Practice '
patience' ,it goes a long way when Smoking. I suggest reading the mods for the ECSS,some on here have good fortune with them,someone will be by and help you.
But,don't give up or get disallutioned , with the Practice and Patients I mentioned, you will catch on;we all started ruining meat when we started.
Hang-on and...