Link Status Problem

Using triple-speed ethernet core. Have erratic, mostly failed, link status. Using SGMII PCS with embedded PHY. Using 1 gigabit in full duplex. Using optical connection to remote test PC. On rare occasions when valid link is established, PCS status register asserts B2 [link_status] and B5 [auto_negotiation_complete], PCS dev_ability and partner_ability registers both assert B14 [ACK] and partner_ability asserts B6 [HD] and B5 [FD], and PCS an_expansion asserts B1 [page_receive] and B0 [link_partner_auto_negotiation_enable]. On the majority of cases when no valid link is established, all of the above bits are zero. Test PC connection lamp is OFF.

When a different [not Altera triple-speed ethernet] optical port is plugged into the test PC, its connection lamp is reliably ON.

Any suggestions regarding debugging this? Thanks.