aggression at the dog park

[dancingdog]

We have a 1.5 year old australian shepherd female. She has a number of doggie friends who she is happy to have stay in her home, share toys with her, and play with her all day. She competes in obedience and is always well behaved around other dogs at competitions and in class. She grew up going to the dog park and playing with every dog she met.
When she turned one year old, she started to guard her toys at the dog park (would bark or growl at other dogs that came near them), so she is no longer allowed to have toys there. We can deal with that. However, not too long after that she started to show aggression (barking or growling, never making real contact) towards some dogs. Many dogs she is fine with. She will either roll over or start doing play bows. Every now and then, she is nasty.
Sometimes I can tell that she might have issues with a particular dog. She hates it when a dog is in her face, hyper and out of control. When I see a dog like this coming over the horizon, I move her in a different direction. However, sometimes I just can't predict how she'll react with dogs that seem fine. I would hate to stop taking her to the park.... and I hate dealing with this behavior. Any suggestions?