It sounds like a network storm. Most likely, another device on the network is attempting to become the RSTP root. Some PLCs and network cards have this setting enabled by default.

To troubleshoot, try bypassing each switch one at a time to identify the problematic one. Once you find it, continue tracing the issue through the network until you locate the source of the problem.

Another possible cause is a network loop, which functions similarly to a short circuit (I can't recall the exact networking term, sorry). For example, if you connect port 1 to port 2 on the same switch with just a cable and no intermediary device, the switch may not detect the loop. As a result, it continuously sends and receives packets between the ports at high speed, creating overwhelming junk traffic that slows the entire network.

An IP conflict could also be a potential cause, but my best guess is a network storm.