I have a yellow lab who is always friendly with people and most other dogs.Three times over the four years that we've had her she has gone after a leashed dog .Obviously she was not on a leash during these scuffles and generally will just ignore or have a friendly passing with other leashed dogs.Is there some kind of training we should do or is it just an instinctual reaction to a certain type of circumstance or vibe she is picking up from another dog?

A highly trained dog can sit and ignore anything until you give the command. You are very lucky you have not been seroiusly sued over a unleashed dog having a scuffle, had the other owner reached to separate and been slightly scratched ever, it can be very serous, or they may do seroius injury to your dog to protect thiers.

It doesn't matter if it is an instinctual thing or not. Your job as a responsible pet owner is to ensure that your dog is not a danger to other dogs or people. If you know that your dog has attacked other dogs on leash, then you need to figure out how to control that behavior. That may very well mean that you do not have your dog off-leash around leashed dogs.

If you do not get this under control, not only can you be sued if your dog does damage another dog, but it may be considered a vicious dog since you know that it has a history of attacking, and it may be ordered destroyed. I'd suggest finding a good trainer or behaviorist to work with so that you can find out what triggers the action and how to handle it.