Possibly Aggressive Puppy

[DMK726]

I have an 11 month old border collie or flat coat retriever mix. Very nice dog generally gets along well with all other dogs and loves people. I take him to our local fenced in dog park (2 acres) where all dogs have to be licensced and up to date on shots. He is about 40 lbs. He becomes somewhat submissive - lays down (does not roll over on back) or will run for shelter under the benches or to the various people sitting around, when other dogs become aggressive with him. Several times a few of the dogs have tried to bite him as we entered the gate and he growls and snaps back at them but does not bite. I have noticed that when he plays with the tug rope with other dogs he seems to get very agitated, so I take the rope away and just don't allow him to play with it. Recently I was told that he had been playing with the rope with another puppy and that he bit the puppy on the leg, ear and along the lip. I am usually very careful about watching him and did not witness this altercation (neither did anyone else) - everyone at the park loves him and considers him a very good natured dog. But, I can not imagine any reason for this dogs owner to say that my dog bit theirs if it had not actually happened. Most times the 2 puppies play together very well - and my dog plays well, but sometimes roughly, with other puppies - when things get out of hand one or the other will yelp and then they go their own ways. My concern is that if my dog is now exhibiting such greater aggression, especially biting the dog in 3 places, that it will become a serious problem. How can I tell and what can I do?