503 Backend Fetch Error intermittently appears

[ocs55x10]

The following URL throws a 503 Backend Fetch Error about half the time I try it.

http://ocs55.x10host.com/education/viewScores.php?exam=Tactics Exam Pretest 1&passPercent=.80

Is there a reason, am I generating too much traffic on this URL? I have free account.