Saint Bernard - Sporadic agression

I have a femail Saint Bernard that I got through St. Bernard Rescue at 8 months.  She is now 4 years about 98 lbs.  I take her to the dog park twice a week and she plays with a couple dogs in the neighorhood so she has been socialized. Everyone loves her as she is a very loving dog.  Problem is that for the last few years she has sporadically become aggressive to others dogs and has attacked them.  Two she hurt.  There is no particular things I can find that she zones in on.  Not the sex, breed, color.  I've started using a shock collar which helps most of the time. Most of the time just having her wear it helps.  But I don't want to have to use it every time I take her out.  


Any one else have this problem or any suggestions? 

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