I had/have the same thing going on. CAC was great, 156.82 a couple of months ago, and then started dropping in the past month pretty fast. It started dropping fast when I didn't let the Roadster balance for 2 commuting trips. Started to take longer than the usual 1/2 hour. So I allowed it to balance 2 hours. Still had to balance more. Then allowed it to balance overnight. The CAC dropped down to as low as 152.83 and has recently bounced up to 153.50. Pulled the logs and brick #90 is the lowest brick that has been there longer than usual. #10 is the next runner up. The average brick of the pack is around 157-158. I know around this time that my CAC drops anyways for whatever reason. My ideal miles when at 157 CAC was charging to 184 ideal miles std, its dropped down to as low as 179, and I'm now at 181 @ 153.12 as of this morning. Will be keeping an eye on it, I can see it rotating that lowest brick out and bouncing back up. To me it seemed like the out of balance really threw the weak brick out of whack.
I'd check to see how balanced your pack is after you allowed it to normally balance each morning, and if its not balanced give it some time, a day or so to fully balance. Then observe.
And yes, get the logs.