Lan limited to 100M-base

[FlameRider]

 I'm having an issue where Lan is limited to 100M-base but when I try to change it. It loses connection entirely

[cookiemonster]

It would be good to tell what is the hardware setup but for now please check loose cables, they have a nasty habit of showing this behaviour when not having a flush seating in the jack.

[slackadelic]

Along with bad cables, could be a bad port on the uplink from the LAN adapter to the switch. 

I've also seen this happen when the cable in question is missing a pair of wires, seen this with some factory cables that weren't designed to work above 100Mbit.  Might try a new cat5e or hire cable and see what happens.