Is it acceptable play to chase an agent from portal to portal?
I'd say abide to local laws.
Might be rather creepy unless you get their feedback on how they feel about it. If I were in the shoes of that agent, I'd be rather irritated that you're following me. I get that it happens when agents from other factions are sometimes right on each others heels, or if you're following an agent from the same faction, but purposefully following someone you do not personally know is not a good thing to do, in my personal opinion.
If you're repairing portals as someone is destroying them, that's called "Playing Ingress". If you're following someone in the car behind, or literally running after them on foot, then there may be a need to dial it back a notch.
Ingress doesn't have Player Vs Player activity, only Player vs Portal activity. Any direct interactions between Agents of different factions should remain friendly and sportsmanlike.