The Link/Act LED should be on solid when data is not being sent and blink when it is.
The Full/Col LED should be on solid most of the time and may blink infrequently. Theoretically, an Ethernet switch with one PC connected to each port should not have collisions, but the Linksys manual says it’s possible.
The 10/100 LED should be orange and on and all of the time unless you are using 10BASET (10 MHz) adapters.
PC1 may have a bad cable or port. To verify look at the LEDs on the network adapters and swap cables/move PCs. Swap ports on the router.
A bad cable may corrupt packets, etc. and cause packet retransmissions, which may appear to be collisions, may operate at 10 MHz and not 100 MHz, and may operate at half vs. full duplex. Larry