I'm just thinking aloud here, but is two one inch exhausts = the area of eight 1/2 inch holes? I'm thinking you may need more exhaust area !
But in the 5 UDS I have built the temp stays +-8 of the seting once the meat is on. Until ash buildup or outside temp or wind changes a lot
Not that I think your exhaust is causing temp issues, I'm more concerned that it may not be as clean or blue as it should be.
Hook Line and Sinker
Viking 30 inch Gravity Fed Charcoal Smoker
Brinkmann 5 burner gas grill with sear & rotisserie
UDS's 3 and growing
Char Griller w/SFB w/mods
Charbroil 7000 Gas Grill