aggressive play

Question
Our 18 month old lab mix is becoming increasingly aggressive when playing with other dogs.  He brings them down, grabs them by the neck or collar and drags them along the ground. He is intensifying this behavior to the point that other dogs are in danger of being hurt.  We have tried stopping this behavior, but it is only becoming more strident.

Answer
Is your dog neutered? Did you get to meet his parents and if yes, how was their personality?
I recommend NOT to take your dog any longer to dog parks/dog play sessions as it only feeds his aggressions and it's simply not fair to the other dogs he's in contact with (I've worked with dogs which encountered traumatizing "Play" at dog parks and such have become fear aggressive because another dog dominated/tackled/attacked them).

I'd recommend finding a very, very good behavioral dog trainer who offers group classes. Group classes are social, constructive and productive. That way he's in a controlled environment and instead acting out aggressive while mingling with other dogs, he has to work, focus on you and at the same time gets desensitized. It is important that he receives discipline and training ASAP.
Good luck!!

Additional thoughts: Do you play tug-of-war with him at home? What type of training do you enforce, like do you emphasize harsh methods like alpha-rolling? Have you had him checked for medical conditions as other dogs often "Know" if something is "off" with another dog and the animal in question will get defensive. Have him checked by a vet, please.