FCGI echo example unstable under load?

I compiled the echo.fcgi example using gcc on a 64bit debian lenny box.

I then used Nginx as a webserver.

Testing the connection shows the FCGI is called and echos fine.

Using apache benchmark with -c 1 -n 1000 works fine and performance is 
good. increasing concurrency to -c 10 causes problems:

on first run, everything may go well. on, say, the second run, the first 
700 requests may go fast but then last few hundred take several seconds.

running again causes same problems but even slower and the application 
may hang/crash (nginx still works fine). in such a case the application 
dies and no data is returned.

does anyone else have the same experience or just me? I'm using 
spawn_fcgi to launch the echo.fcgi process.