Hi Mircea,
Is this for a MangoES? If so, the following link may help: https://wiki.debian.org/NetworkConfiguration#Setting_the_speed_and_duplex
As noted in that link, do be careful (particularly if this device is remote!) that you do not inadvertently lock yourself out of the device. Unstable link speeds can often be symptomatic of bad cabling, faulty termination, or intermittent noise and identifying and addressing the root cause may be called for.
One other thing to note is that many switches and other bits of network gear allow you to set a 'manual' speed by effectively disabling some rates, eg. you could disable Gigabit on the switch port and auto-negotiation would max out at 100Base-TX.