"WARNING: TX SGDMA TImeout" and no data sent

Interesting. I am having a similar problem - except that the transmit sgdma never finishes (always busy), and the receive FIFO is never emptied.

Have you looked at the 'MAC Configuration Register Space' to see if the FIFO (transmit in your case) is filling up? (You might also find interesting things in those registers as well - a perusal of the options and their values may shed some light on the situation)