Did you install a bunch of plugins that trigger effects when a user registers (like badge, top users, coins, avatar, privileges, extra fields, spam prevention...)? Are there some conflicts among them? Are they implemented properly (for example, a random avatar plugin with image folder but you put a ZIP file in there)? Do those plugins or core hacks create an endless loop? The worst scenario is that your site got malwares. Some fake plugins or injected malicious codes may do evil tricks with database.
BTW, remote databases are generally a bad idea for applications that do lots of queries like Q2A.