After a week or two of daily use, I feel Iron v184.108.40.206 (30207) is more stable than v3, and not much slower (Windows Vista on Core2 Duo 2GHz w/ 2GB RAM).
Two serious issues:
1) Though less frequent, the crashes continue where while waiting for a page load the UI stops responding one feature at a time (first links and drop-downs, then a moment later scroll bars, then keyboard response). When Iron is closed on these occasions, it leaves processes running in memory, at least one of which cannot be shut down with Windows Task Manager. When restarted, Iron may or may not be able to access the internet, and then I must restart the machine to use the web.
2) Was stuck in a loop with Gmail where Gmail kept automatically signing me out "for security", possibly because "this browser was recently used to sign into a different account" (not true). Clearing browsing data solved this, but took several minutes.