SM is installed on Windows 2012 Essentials. I'm running the Windows DNS server and in SM, left the primary and secondary DNS server blank so that it will use Windows. The Windows DNS server itself is using the root hints and has no forwarders configured.
Every couple hours, the average resolve time for Spamhaus will go from ~500ms to ~150,000ms and counting. When that happens, querying dbl.spamhaus.org via nslookup will time out. When that happens, I'll fire up the Windows DNS MMC, right click on Cached Lookups and select Clear Cache. Afterwards, it immediately starts resolving again. This is the only domain name this happens to, everything else, including all my other RBLs, resolve fine.
My backup MX server, which is setup similarly (albeit on Windows 2003), doesn't seem to have this issue. I'll update this later today and report my results.
I haven't rebooted my server in awhile, so I'll try that first. If that doesn't yield results, I'm going to experiment and tell my primary SM to use the other Windows 2003 DNS server and see if it has the same issue.