A big part of what is going on is there is one huge miner in the pool. When their payouts hit the queue, that can be multiple blocks, and if you're behind that, you will have to wait. For example, the most recent payout to the big miner is 118 BTC -- more than 4 blocks worth. I guess that must have been done manually to clear the queue since that's more than one block, although I don't quite understand how that works in the CPPSRB system. Their typical payments are around a full block, which means no one else gets paid (much) for that block, and the queue backs up a bit. On the other hand, when they don't have a payout in the front of the queue, their hash power will help find blocks faster and speed up everyone else's payments.

So with those huge payments going into the queue, things get more irregular, sometimes faster, sometimes slower.