Is this a memory leak issue with DNS or a physical memory limitation with the new Kernel? With any DNS cache algorithm, you should be aging DNS entries and pruning that cache frequently. This is especially true when you have limited memory, like on these little routers.
So, to me this looks like some fundamental issue with memory management on the box. This is especially true if by disabling the DNS relay, it does not clear up the issues for everyone.
My guess is that there is a general memory problem, and DNS is the largest consumer of available user/system memory.
TheWitness