Got a Humphrey's Pint and have some questions

[HeyMistah]

Long time lurker first time poster. I recently got my Humphrey's Pint and it is beautiful. This is my first vertical smoker, used to use a UDS. I know there will be a learning curve, but I thought I would get some advice from some experts. I researched around and here and saw some helpful tips. I still have issues climbing/holding temperature. I filled the tray 3/4 with charcoal (Kingsford) and put a layer of briquettes in the first 1/4. I lit about half a chimney smoker and put that into the first 1/4. I kept the firebox open, closed the side valve and let it climb. My version does not have the daisy wheel in the front. When the temperature was 50 degrees away from the gasket set temp of 225, I closed the firebox and turned on my guru (cyberq wifi w/ pitviper fan) and opened the valve. The temperature stopped at 175 and began to fall and kept falling. It got down to 135 and that's when I went out there and opened the firebox and it the coals look dead. Blow on them and no orange glow or anything. So i took all the burnt coals out and started over. This time I let it climb higher and then closed the fire box. I hit 225 but only for about 15 mins before the temperature began to fall again. Right now the temperature is at 217 guru output 100% and the temperature is still dropping. Any suggestions? Thank you in advance!