Slow speed on the PC port

[jolouis]

Perhaps even a more basic question needs to be asked here:
What is your cabling infrastructure, and are you 100% sure that it is installed and operating as per industry standards?

99% of the time we find customers that simply drop VoIP installations onto existing offices/buildings without even bothering to check and make sure the network wiring was done properly in the first place. A PC can drop packets and toggle link speeds without anyone noticing, but when you put a phone on it that's a whole different ball game. Since you have already stated that replacing the phones does not make a difference, I would suggest having a proper cabling company come in to check the wiring (or at least moving one of the "suspect phones" directly to the switch in the LAN room, and seeing if the problem goes away when it's there.

You would not believe how many people in the world think that the color code and termination standards for Cat.5e/Cat.6 are just "Arbitrary recommendations" or do not know about them at all lol End of the day though get a proper cable test done and I bet you will find the source of the problems there (either wrong wiring, loose pin/punch down, crosstalk or some other physical fault).

The phones are probably negotiating to 100M, but then over time as packets drop and strange things happen on the link, they end up falling back to 10M or even half duplex.