Check your thermometer by putting it in boiling water to see if it reads what it should for your elevation. In Denver it should boil around 202 F. You can put a stack on your lid but it is unnecessary. You likely do have an airflow issue. Make sure your intake isn't plugged with ash or something and make sure the thermometer isn't touching your grate or the food and that it is under the grate between the grate and the fire. An inch under is about right.
I cooked some ribs yesterday on mine. Lump charcoal lit on the gas starter on my performer grill in a weber chimney. I filled the chimney 3/4 full and put some pecan wood chunks on top of the coals. When the wood was starting to catch fire I dumped it. My lid has the small and large bung hole with no exhaust stack. I just remove the cap from the large bung for exhaust and open my 3/4" ball valve about 80 percent open. It took ten minutes to light the charcoal and when i dumped my smoker was at 250F within a couple minutes. ten minutes later my smoke starting turning blue and I threw the ribs on. The temp dipped to 220F but was back to 250 fifteen minutes later. My ribs were cold actually frozen (an experiment) that caused the temp drop.