If this is a MTU problem, you can determine maximum MTU by issuing this command:
/ping <<your GW ip>> do-not-fragment size=1500
And reducing size variable by one each time. Notice the MTU when ping will run, it will be your maximum MTU. 
Similar problem here: Can’t open a webpage, not just AIM not working.
This was my first idea too, but the manual states clearly on ECMP that it works with pairs of source-destination-IP-addresses.
Could someone take a closer look to it with a packet-analyzer? I do not have two upstreams available at the moment.