SNAT accepts a --random option (from iptables-extensions manpage):

--random
       If  option --random is used then port mapping will be randomized
       (kernel >= 2.6.21).

So I'd try something like:

iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to-source 193.49.142.107:2000-4000 --random

SNAT accepts a --random option (from iptables-extensions manpage):

--random
       If  option --random is used then port mapping will be randomized
       (kernel >= 2.6.21).

So I'd try something like:

iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to-source 193.49.142.107:2000-4000 --random

SNAT accepts a --random option (from iptables-extensions manpage):

--random
       If  option --random is used then port mapping will be randomized
       (kernel >= 2.6.21).

So I'd try something like:

iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to-source 193.49.142.107:2000-4000 --random

Disclaimer: I haven't tried this myself.

SNAT accepts a --random option (from iptables-extensions manpage):

--random
       If  option --random is used then port mapping will be randomized
       (kernel >= 2.6.21).

So I'd try something like:

iptables -t nat -A POSTROUTING -o eth1 -j SNAT --to-source 193.49.142.107:2000-4000 --random