MS510TXPP Throughput Issue

Hello all,

I have a pair of MS510TXPPs, each connected to a router via 10G SFP, and 300Mbps internet, and for a long time all was well - one MS510TX was set up as a dumb switch, the other did some light VLAN retagging and forwarding, and running a speed test showed I was getting about 300Mbps.

Then I upgraded to 1Gbps internet, and suddenly I cannot get more than 300Mbps through the MS510TX's - I can connect to the ONT and get 1Gbps, I can connect to the router and get 1G, I can connect to the SFP+ port the MS510TX is using as the uplink and still get 1Gbps, but any time either of the MS510TX is in the chain my performance drops to 300Mbps, almost as though it's hitting a limiter. The ports are all negotiating to the proper speeds (my test machine is 2.5 Gbps), I am running the latest firmware (6.7.0.52), I have factory reset both switches, I tried disabling routing functionality just so it's not hitting the CPU, and every once in a blue moon when I run a speed test I do actually see 700, 800 Mbps through the switch, but upon repeat it goes back to 300 Mbps. 

it appears as though I am not the only one with this issue (here and here, for instance), but I don't see any sort of resolution (save one poster recommending returning the unit). Anyone run across something similar, or know of a reason why a factory default MS510TXPP would have a throughput limitation like this?

Thanks in advance.