no its not ports time individualy, just ip changes on through devices
and as i said it might be at this point(im not sure) :
and since you mentioned they drop randomly maybe it switches ip each time ?
and you just set 1 ip and 1 ports to 1 xbox seperately and it just jumps to other ip which doesnt have ports set.
and once again some routers do have this.
ps. you might try doing this :
1. disconect every posible device which might use router
2. turn on both xboxes and set them as they should be
3. connect and start playing.
4. if you get succes for some time, try turning on wifi on your mobile phone for example.
5. if this will dc one of your xbox then its the ip changing problem.
iam posting this posabillity since i had myself once, and i literally over came it with setting multiply ports on each new ip individually,
in your case you should find something which might be causing you on ip changes ?
i hope it helps.... good luck