Resource temporarily unavailable

I have a servlet that generates a framesetpage. Each of the three frames on
that framesetpage is also a link to a the same servlet, only with other
parameters. The framesetpage has two parts: a topframe and a bottomframe.
The bottomframe has again two parts: a leftframe and a rightframe. The
topframe gets data from two tables, so does the left and the right. They
each make a connection and a resultset for each (6 of them) query based on
those tables. Additionally, they use a common table for metadata.
After a few refreshes(three or four), he gives me an error: resource
temporarily unavailable. How can I resolve this issue? Sometimes, I don't
even have to refresh to get the error. The server my servlet is running
under has a threadpool. But how do I prevent this error from occurring
again? I don't need much, but I have to at least support 5 users at a time.
I have tought about threading or connection pools, but I know very little
about it.

Any help is appreciated.
Nico.