UART transmission drops characters

[9names]

There's something wrong with the current UART free-running configuration or implementation.
When transmitting, we sometimes will drop a single character.
This happens quite often, but not at regular intervals.
Sometimes we get a few thousand characters with no error, sometimes it happens after 100 chars.

Things I have tried to fix this:

• Adjusting how we use the FIFO (only pushing data in when empty)
• Setting FIFO thresholds to different values