If your Reaper session runs one day perfectly fine, then the day after not,
i would say all the audio device / samplerate / buffering settings in Reaper are just fine and Reaper is not the culprit, something else on your system is to blame.
That spiking kernel task is interesting ..
Do you maybe experience on the next day heavy spinning of the fans in your macbook ?
Also as sidenote: i am not sure if the thunderbolt <> firewire connection is a really trustful and stable solution, although i do understand, you have no other choice.
Do you use a high quality cable for this connection ?
There are some really good Apple knowledged people here in the Community,who maybe can also help you further with your issue.