Thanks for all your help!
I think I've "fixed" it.
I connected it up in another vehicle using the same supplied power adapter and it completed the self test. I immediately did a factory reset, then reconnected it back up in my car and it has been working fine for the last few days, Fingers crossed!
Incorrect Speed Warning in both direction at the following location. There is NOT a speed camera anywhere in the area. I'm running v1.50 with the July GPS data. Previous version was the same problem.