Major slowdown on wired LAN

I’m very puzzled. The one problem I’ve not been having is transfer speed. 80-90 mbytes/second typically.

That’s changed now. Everything starts fast, then slows down to essentially zero speed.
I have restarted the Drobo, that doesn’t help.
Restarting the PC helps, but only for a few minutes, then everything’s slow again.

I’m running a 5N2 with port bonding enabled, each port connects to a different gigabit switch.
While the slowdown is happening I can still access the Drobo with dashboard though it’s a little slugginsh, even when file transfers are essentially stopped. It seems like an issue on the PC side, but it only happens when talking to the Drobo.

I looked online and all I found was stuff about autotune, defragging the hard drive, etc which I’ve tried to no avail. I still have the problem working from an SSD, with a 5 GHz CPU and 10 gigabit ethernet card.

Where can I look?

Have you looked in the Windows event logs? There are often clues there to what happened when some issue occurred.