Yeah when I want 225 I'll only light and add about 6-8 briquettes or enough to JUST cover the wires at the bottom of the chimney to my fuel basket. I've never added a full chimney. Sounds like with that much lit coals, you're lighting too much other coals from them throughout the cook. Try and adding only a few coals and slowly bring it up to 225 and then shut down the intakes to get it to cruise. Some drums just have a sweet spot that's hard to fight though.
I've never tried the water pan in my UDS. It works well with the ECB, but that's like a WSM and is designed as a water smoker. The UDS is not.
BUT, as Bo said.....fiind what works for you. Give it a try and see if that helps.
Big JT's Smokin' BBQ Competition Team
"The path was worn and slippery. My foot slipped from underneath me, knocking the other out of the way, but I recovered and said to myself, 'It's a slip and not a fall.'"