Stlitz, I doubt the basket is the problem. I bought a replacement water pan and punched a bunch of 3/4" holes in it for a charcoal basket for a friends UDS and it has no problem with the temps.
If it was ash build up, it would affect the later part of the cook and not the entire cook.
All that being said, if you have a step bit it wouldn't hurt to try opening up some of those holes.
And all that again being said, if it is steady 220 the entire time and it doesn't spike, I would use the old computer programmer saying, "That's not a bug, it's a feature!"
Wannabe BBQ Illuminati