Tired of Tough Brisket !!

I have a old stick burner made out of 3/8 "thick/24" pipe. The cooking area is 4'6" and I have a 30" fire box. I can hold the heat pretty constant ( +- 10deg.) and have cooked brisket from an hour/pound to 1 1/2 hr./ pound and have wrapped it from 2 hrs. to 5 hrs. into the cook time. I still have a major problem getting the dang stuff tender. What am I doing wrong?? Ribs, chicken etc. is great, but brisket *%$#^&&^T%!!!