nat (inside) 2 10.5.1.0 255.255.255.0
global (outside) 2 172.16.5.1-172.16.5.30
You can decrease the xlate timeout on the pix with
timeout xlate 1:00:00
default is 3 hours. Do "sh timeout" to see defaults.
Bear in mind that the timeout settings on pix v6.x are global so this will affect all xlates.
Jon