How do I modify the uart ISR?

[Beowulff]

I have some very tight serial timing requirements. Once I receive a command byte, I need to respond within 2mS. By setting the FIFO Full level to 1, I am able to do this most of the time, but there are still occasional delays that cause the response time to exceed my requirements. I would like to modify the uart ISR and add a global tick counter, so that I can tell exactly when a byte is received. This way, if there is a delay in responding to the interrupt, I can ignore the command, and wait for another one to be received. But, the ISR is in a library. I tried to copy the code and modify it, but there are way to many dependencies, and I couldn't get the code to run without crashing (I think it has to do with the large uart structure that is being passed into the ISR, and not passed to any other substitute ISR).

Any idea on how to do this?