I will throw you 1 bone, but past that, I'm careful because my work here is not my own, it's my employers.
libxml_use_internal_errors(true);
This little function call is the secret to solving what seem at first to be intractable memory-leaks. The trouble is the scraper uses libxml, and libxml issues a notice/warning every time the HTML is malformed. Without this call, those errs will bubble up to the PHP error handler and it'll murder performance and memory usage.
One more i suppose...
If you scrape inside a loop (and unless you're using a distributed job queue,if you're scraping more than one URL at a time you almost certainly are), missing a unloadDocument() call is going to cost you each time you iterate. The objects it creates, IIRC, have some circular dependency issues and if you don't explicitly unloadDocument you'll run into trouble. (Suppose it should be OK tho if you've enabled that GC feature in PHP 5.3)
And, generally, a tip... sometimes it's tempting to write a simple regex instead of a chain like, say, pq($this->node->find('a')->get(0))->attr('href')
Write the chain. Regex is just too brittle.