policy routing and NAT

I use a multihomed enviroment on a Linux-box with 2.4.22 and iptable
1.2.8 which stateful inspection.

Internal eth0: 192.168.1.0/24
external 1: ppp0 with staic $IP1 gateway: $GW1
external 2: ppp2 with dynamic $IP2 gateway $GW2

I want to use policy routing with the following rules (this is only a
example)

ip rule add fwmark 5 table t1.out
ip rule add fwmark 6 table t2.out
ip route add default via $GW1 src $IP1 table t1.out
ip route add default via $GW2 src $IP2 table t2.out

ip route add default via $GW1

iptable -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
iptable -t nat -A POSTROUTING -o ppp2 -j MASQUERADE

iptable -t mangle -A PREROUTING -p icmp --icmp-type ping -j MARK
--setmark 6
iptable -t mangle -A OUTPUT -p icmp --icmp-type ping -j MARK
--setmark 6

This works fine for all forwarded traffic, but every icmp-traffic
originated by the router itself goes out to ppp2 with the source IP
$IP1.

What have I to change, that the source IP for outgoing traffic on ppp2
and originated from the router will be changed to $IP2 (the address of
ppp2)?