Hi!
configure forwarding ipmc lookup-key group-vlan used to be more of a pain with the X440 non G2, but surely it can affect the G2 too. It does have very small tables even if they are magnitudes better than the old X440... I'd start monitoring the switches with some ping tracer to see where the problem starts. Do you have in-band management on the X440G2s? If so, setting up smokeping (made by the same guy that made MRTG and RRD, Tobi Oetriker) on a Linux PC (or VM) could let you trace where and when the problems occur. You have a lot of work in front of you finding the cause, but if you're lucky, tools like this can help. A TAC case will certainly be helpful too as suggested.
Simply pinging a lot of switches (if management is in-band) might be a good starting point, but smokeping is my favourite! Remember to ping with a size of 1472 bytes as that will produce IP packets with 1500 bytes size. If you use smaller paket size, MTU problems may not be detected (not that this seems to be the case here, but...).
Check optical levels if you have fiber links. "show ports transceiver information".
Check for RX errors (and collisions and TX errors while you're at it), mainly on uplinks. You should have a tool to monitor that along with utilization.
Check for congestion (tail drops, buffers running over) "show ports qosmonitor congestion"
You'll be surprised to see how soon some switches drop packets. I've seen heavy tail drops on X460 (I think) with 22 % utilization (as reported by the CLI 2 second update).
/Fredrik