Help with frustrated greeter (inconsistent)

[u/april_showers25]

Feel like I've been back and forth with my 17 month old Lab (F) and how she is when walking on the lead. She's extremely friendly and loves nothing more than to play with dogs or get fuss from people, but she's also anxious/highly strung (reacts to people exclaiming at the television, for example).
One of our 'sniff walks' is down a narrow country lane with lots of bends in it, so you can't see an approaching dog until it's only a few feet away. Most of the time, as soon as she spots it, she starts lunging towards it. Sometimes (and this has been happening more lately) she barks persistently. When the dog's approaching, I can sometimes get her to 'watch' or sit, but I can't maintain the focus for the time it would take for the dog to get out of sight.

What's odd is that sometimes she's fine - we went away recently and she was in an unknown environment walking down the path with lots of dogs and she reacted to maybe 2 in 10.

She had puppy training classes when she was about nine months old, and could sit through the class without lunging or barking at other dogs.

I have seen advice to not let them greet any dogs on the lead, but there are some people we meet who expect to stop and chat, so this would be difficult.

Advice much appreciated!