Just to confirm you are using the power lead provided with the PilotAware classic unit ?
Don't I feel a numpty?
Lee, I read the above and immediately thought "Yes, naturally: I've definitely been using the supplied power lead" and then.... "Hmm. Have I? I think so but..."
Anyway, I made sure to use it just now and sure enough the device no longer falls into a reboot cycle - problem solved. I've successfully started half a dozen times now with no issues.
So lesson learned: Use the supplied lead, even if you think an alternative is up to the job...
I've marked the lead up as "For PAW" to avoid using an incorrect one again.