Client has to wait long for its answer after many queries served,although server is still fast

Dear all,

I have the following setup (OS is Linux 2.6.16.43.1.amd64-smp, Debian
Sarge):

1. A server written in C++: It waits for a request on a given port
(using accept), reads whatever is sent (using read), computes an
answer string, sends it back (using write), then closes the connection
(using close).

2. A client written in PHP: It opens the connection (using fsockopen),
sends the request string (using fwrite), reads the data (using fgets),
then closes the connection (using fclose).

Queries come in at an average rate of about one query per second. The
server needs a few milliseconds to compute the answer string

The whole thing works fine for the first, say, 10,000 queries.

Then, for some reason that I don't understand, it happens more and
more often that the client has to wait several hundred milliseconds
before it receives the first data.

Everything is still fine on the server side: computing and sending
still just takes a few milliseconds.

When I restart the server everything is back to normal again. The
phenomenon also happens when only few queries have been processed, but
rarely. After the aforementioned 10,000 queries it becomes the rule
rather than the exception, but there are still queries for which
everything is fine then.

Any ideas what might be happening here?

thanks for your help,

Holger