Multiple dnat for multiple adsl

Hi guys

Sorry for my bad english :p
I have a problem with a customer.
This customer have a network quite complicated with 2 ASG 325 ,4 adsl an multible LAN with a DMZ with multiple server.
All the user surfing in the web with the adsl number 1,the default gateway,and work fine.
For the inbound traffic era many dnat and firewal rule in "failover" in the other 3 adsl, all of them work fine to watching the live log.
The problem is, a correct entry to the "service 1"the first time fail,the same entry to the "service 2" work fine and after the same entry to the "service 1" work fine....

I can solvig with uplink balancing and multipach rule with persistance by source and destination together ?
Or is better a persistance by interface ?
Or is a problem from proxy arp ?

Thanks

Luca