Apache2, FCGI, 500 errors?

Context: shared server (DreamHost)

Does anybody know why these happen at random moments?
The http logs say:

…[error] [client] FastCGI: incomplete headers (0
bytes) received from server…

But the request before it works fine, and the request after it works
fine. I’m currently running three Rails apps on this server. I ping them
each every ten minutes. I get a 500 error about 4-8 times every 24 hours
from each. There’s no perceptible pattern, which leads me to think it’s
outside my control.

I particularly noticed this when adding an upload progress bar to an
app. It works fine, but occasionally I get the error message instead of
a progress update. And I’d really rather not:-)

So if anybody has a clue, I’d like to hear about it!

–Al Evans