Father's Day UDS problem

Smoked a butt yesterday and had my first problem with my new uds. Previously I did a 9 hour test run with no food and a five hour smoke on a couple of slabs of ribs. Both times holding temp was no problem, maybe not perfect, but always staying in a 3 to 4 degree range. Yesterday I started it up and got it settled in around 250. About four hours in, the temp suddenly dropped like 50 degrees in in a five minutes span and seemed to be headed further down. I opened the intake vents a little and took the top off for a few minutes to get the temp back up. It went for another hour and a half and then it suddenly dropped again. From there on out I had to have all three intakes (3/4 in nipples ) open all the way to keep it at 250. In the end it stayed at that temp until the butt was done and it actually came out awesome, but I don't like that I had to have intakes wide open just to keep it at 250, and that wasn't a problem either of the first times I ran it. Thoughts? Suggestions?

Well, you have correctly guessed the charcoal I was using! Thanks for the info. I will look at other charcoal options, but I haven't seen a lot of variety by me. If it happens again I'll give it a good kick/shake and see if it fixes the problem. Thanks again.