I do like the Zeftronic VR. Unfortunately I had some minor issues, but they were cleared up. Tech support was excellent.
But, just for a 'heads-up', grounding seems to be the root of most electrical problems. Our planes are 50+ years old. It's not a bad idea to spend a little time cleaning the grounding contact areas.
Just my experience!
I've had the same experience with the Zeftronic regulator. I even replaced it with a new one, and it still was cutting out after about an hour of flight. The Zeftronic tech support guy suggested I make sure all the contacts to it were tight and clean, to prevent a voltage spike on the overvoltage sensing circuit. I did that to the best of my ability, to no avail. A friend suggested looking at the ground path. That rang very true to my experience, so I looked it over.
The factory-installed ground has a lot of links to it: from battery through a braid to the firewall, then an aluminum strap across the shock mount to the engine mount, then jumping over another mount to the engine. There are a lot of opportunities there for connections to deteriorate, and while this setup might have been just fine for a new 170 as equipped from the factory, it might not be good enough any more in a 55 year old airplane with advanced electronics.
I added a second cable from the battery terminal to one of the starter mounting studs, and that seems to have solved the problem. This only addresses the ground path from the battery to the engine, so I think it really only helps the starter, generator, and voltage regulator. If the avionics are spazzing out, the problem has got to be somewhere else.
I really like the Zeftronic units, and their documentation and tech support are excellent. The later units have LEDs on them that help you diagnose problems. I installed a new one in my Swift this year after an overcharging episode with a relay-based regulator, and feel much better about the whole system.