Hi Jon, you are not the only developer experiencing this issue.
A few days ago another developer was reporting the same issue here:
I also heard from another developer that their integrations have been experiencing Token drop outs as well. They have to re-authorise the connection every so often.
I recommend that you create a Developer Support Ticket to report the issue.
https://developer.infusionsoft.com/support/tickets/create/
Keap API team (@TomScott) need to investigate the issue because there seems to be a problem with the OAuth Token mechanism.