trip mileage meter goes to zero upon staetup, no matter what has been the previous reading. Happens on both one and two.

 

That's one that I have never experienced. My suggestion is to pull the cluster and send it to a competent repair shop. There are dozens of them and if they advertise Volvo clusters, they have the appropriate tools to do the job.

I've seen one such place in operation. They plug into the cluster and send the same signals that the vehicle gets to diagnose the problem. You may have a stuck micro switch which they would replace.