For the internal traffic you will need to use "u-turn" nat. the issue is that because the source and final nat destination are in the same subnet, the traffic is asymmetrical. the reply goes directly to the sending computer instread of back to the firewall state table where the destination nat occured.
To correct this on your policy from the internal host to the external ip address add a source nat interface to the policy. this will force the traffic back to the firewall.
edit policy > advanced tab > add source nat interface