GA11-15 (2002 model) serial no. AII2691AO

The compressor stops loading randomly. It gives us warning signal, when we look at the status, it only tells us protection alarm. Not sure which one specifically. We checked the ca/co temp, it's at around 80 and shut down set at 120. The LAT is at 16. We have overhauled the unloading valve using the original Atlas copco repair kit, but we noticed the the piston (1613 7031 00) has gouges which look like the piston rubs on the spring. We tried replacing the loading solenoid valve as well and the problem still exists. Could the problem be in the unloading valve that's why there are gouges on the piston? Or it maybe the solenoid. Is there a relay that controls this solenoid that might have developed scales on the contacts therefore getting stuck? Your kindest advice will be highly appreciated. thanks and regards, Joey de Grano