TSE (Ethernet) - Core does not transmit data on RGMII TX, rest works perfectly fine

I am having an issue where Tx data is corrupted intermittently. Most of the time the Rx and Tx work fine but after a while I get dropped packets due to an FCS error. This may not be the same problem but could be related.

Have you looked at the RGMII signals with a scope to check that there is definitely no data being output? (it may be being corrupted during transmission over the RGMII which will cause an FCS error and your receiver will probably reject the packet with no evidence it was ever sent). Also look at the clock output and Tx control signals to check for activity.

If data is being output you can put the PHY into loopback mode to analyse the Tx packets.