I've had the exact same experience with my new Baby Jambo. What I've come up with, right or wrong is that when I get to the point when the original charcoal starts to run out, I'll add a second split to the fire. It usually runs the temp up about 25* for a little while (15~ish minutes)but it will give me more coals from the wood to counter the fact that I'm about out of charcoal. From that point forward, I'll start adding the splits with a little more frequency than I was when i was setting them on the bed of charcoals to ensure that they have time to get burning before the old coals run out. I usually keep one split at the front of the firebox, so it will be hot, in case I need an "emergency" split should I miss adding one earlier. That one will be really preheated and will catch fast should I need a quick infusion of heat. Again, don't know if it's right, but it seems to be working.
All that said, my pit likes right around 250. I think if I was to try to hold a steady 225, I'd have a harder time managing the fire to keep sufficient coals without getting a temp spike to 275. Hmmmm. Maybe I should try that this week....
UDS / Jambo Backyard / Red Thermopen