it's just possible ? maybe?
that you have a damaged ethernet cable
At 10Mb the packet error correction can maintain the connection by re-requesting data packets but at 100Mb maybe it just can't keep up ?
Or you damaged the crad when installing your new fan?
Either way a cable is about us$5 and a NIC about the same I would guess, both a cheap fix.
before you do that though as unlovedwarrior suggested uninstall & reinstall your drivers incase they are corrupt.
steve