load balancing, fail-over

I have set up an advanced linux router implementing load balancing and
failover using Julian Anastasov's patch and following Christoph Simon's
Nano-Howto to use more than one independent internet connection.

The router is load balancing between 2 external independent Internet
lines. I have written a PERL script to check if a connection is down.
If a connection is down, the script will bring down the corresponding
interface. It later checks if the connection is back up by bringing up
the down interface with a metric of 19 and pinging 3 different public
websites through that interface. If all 3 pings are successful then it
changes the metric to 0. If it fails, the script will bring the
interface back down.The problem I'm having is when the down interface
is brought back up by the script, new sessions intiated by a web
browser from the local area network stall. However, connection through
the other working interface is fine.

In addition, bringing up the down interface by hand from the command
prompt using the ifup command works fine versus if the script executes
it using the PERL's system command.