Consistent temps on WSM

This weekend i tried smoking soome ribs/rib tips for my very first time. Overall it was a great experience and i cant wait until I smoke something again! The ribs came out...edible but they were not very tender. Im sure there are a lot of factors to this but I have a feeling temps were the main reason. I tried to keep it at around 235 but it would drop to 220 and raise to about 250 at times. I would adjust the vents accordingly but throughout the whole cook. I also think that is what made it take to so long (7 hrs!). Any tips would be appreciated.