Are you aware about broadcast storms? Perhaps you somehow accidentally introduced a loop in the network? The symptoms fit that exactly. https://en.wikipedia.org/wiki/Broadcast_storm
It could be missing IGMP Snooping Protocol support in a network with IPTV or custom VLAN setups. There are 3 versions (IGMP snooping (v1, v2, and v3)), managed switches have them all, unmanaged usually don't have them. To avoid problems, only pass a single VLAN to the unmanaged switch (it must be behind the managed switch for that), otherwise the unmanaged switch can and usually will bring a network down after some time. Or just use a switch with IGMP snooping support.
STP is meant to prevent that. https://en.wikipedia.org/wiki/Spanning_Tree_Protocol
Of course you can't set up STP with unmanaged switches, so until you go managed and set up STP properly nothing will change.