Iptables and SNAT
Remove the three rules you have above and try adding this:
$IPTABLES -t NAT -A POSTROUTING -s 192.x.y.a -j SNAT 192.x.y.b
$IPTABLES -A FORWARD -s 192.x.y.a -J ACCEPT
And enter the following command:
echo 1 > /proc/sys/net/ipv4/ip_forward
You should also have a rule similar to the following in your firewall:
$IPTABLES -t FILTER -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
That will take care of all established/related connections, instead of making a rule for each one.