Confusing server 500 errors - stub start error

I have a perplexing error that I can't figure out. I'm running a server using Sun Web Server 7 and PHP 5.x, and have been doing so since late October 2007. Now, just recently - in the past month and a half - I've seen an exponential increase in server 500 errors, specifically "stub start" errors. Google has very little at all to return on stub start errors, and looking into the Sun documentation has been fruitless too. So i'm wondering if anyone here has any experience with such errors or can give me any tips on what to try next. The weird thing is that the 500 errors tend to happen in spikes - 5,000 of them will happen within a 10 minute span, and then back down to normal levels.