@shibli 

probably faulty cable or connectors. what happened if you manually lock switch port to 1000Mbps?

  @shibli 

what type of switch do you have?

  @MR.S I tried with 2 POE switches. TP-Link TL-SG1210P & D-Link DGS-F1010P-E. Both end up with the same problem. When the AP starts, it gets 1000mb link speed. As long as it stays this uplink, data transfer rate to wifi-ethernet is great. But after 5-10 hours link speed falls back to 100mb and wifi-ethernet transfer rate becomes very low. If I restart the AP manually, it again gets 1000mb and falls back to 100mb after several hours.

  @shibli 

ok, both are unmanaged switches, it is not possible to set the speed manually on the EAP660 so it has to be done on the switch,
there is a difference between EAP245 and EAP660, EAP245 is 802.3af EAP660 must have 802.3at, so EAP660 draws more power, it also has a 2.5 GB interface. but it's hard to say, I still think it's something with the cable or connector,

this is a screen shot of a EAP660 LAN statistic, do you have any error on your device?

  @MR.S AP uptime 10hr & 48min. No error is showing. Both POE switches support 802.3af.

  @shibli 

You have 31 RX errors.. non of my access point have RX or TX error.

This have 53 day uptime en  with almost 2000gb data transfer and still no errors.

 try to connect access point to switch with a short patch cable and se if the same happened.

I don't think the problem is AP,