Have you checked the accuracy of your thermometer? A pan of boiling water is 212°, depending on your altitude. Makes it easy to check the accuracy within a degree or so.
I don't have your smoker, so some of this may not apply.
Try filling your water pan 1/2 to 3/4 full of sand. Cover the sand with foil to catch drippings. The sand (or water) acts as a heat sink to help regulate the temperature of your pit. Helps moderate those peaks and dips as you open the pit to add fuel, etc.
Try a test run without any meat. Fill your charcoal pan with a full chimney of lit coals. Open all vents wide open. Fill your water pan with boiling water. How high does the temperature go? Still can't get to the desired range? Buy a small bag of Kingsford charcoal and try it. Do you get the same results? Since lump usually burns hotter, I'm wondering if your lump may be damp.
Just some thoughts that may be useful. If this doesn't help, you may want to take a look at the archives at http://www.rbjb.com/rbjb/archives/archives.htm
or use the search feature on that site. Look for discussions on modifications that should improve performance for you.
Hope this is helpful.