Thanks everyone for all your help so far (for responding to quickly to all my questions as well as for years worths of everyones contributions - questions and advice givers).  I'm addicted to this site. I've spent so many hours pouring over everyone's posts and answers.

We've had Ralphie for about 2 months now and he is just shy of 8 months old. We've been taking him to a puppy class as well as dog parks several times a week.  Lately I've noticed Ralphie's play behavior is growing increasingly aggressive.  He's even pinned down a sweet 1 yr old pit bull (I was shocked as I've never seen him do this to any dog!).  Today for the first time he was biting the back of this dachsund's neck at the dog park.  I did not like this at all and apologized to the owner and left with Ralphie (maybe a bit reactive but we were ready to leave since we were there for over 2 hours).  The dachsund did not enjoy this it seemed (who would?). Now that I think back I probably should have just said "No" and pull them apart slightly each time it happened but it was difficult since they were not playing just in one spot but running around etc.

Earlier today prior to the park, we were in puppy class and this other dog was doing the same to Ralphie.  This other dog was mounting and biting the back of Ralphie's neck and not letting go. Ralphie didn't seem to like it and he kept trying to get out of the dog's bite.  Is this acceptable behavior or should it be discouraged?  What should I do next time?  I much more preferred his old ways of playing which didnt involve actually biting down on skin.  

When Ralphie was getting bit on the back of his neck in puppy class, I tried to intervene by saying "gentle" or "off" ( I read that you should just let them sort it out for themselves, but for how long do u watch before you break it up?) but the other dog kept mounting and biting.  Ralphie just tried to air bite the other dog by twisting his neck to either side but was never successful.  I still can't figure out when I should intervene as I heard they can sort it out for themselves.  I have been trying to stay out of it most of the time, but now that I'm noticing his once gentle behavior is becoming more aggressive I'm uncomfortable letting him get bit on the back of the neck/being mounted and then becoming defensive and getting more mouthy.  Am I overly sheltering him?  Should I let him tough it out?  But I really loved my gentle, wouldn't hurt a fly playfulness that Ralphie once had.  He used to just pounce, chase and neck rub.

I'm sure some parents will think I'm crazy and say let dogs be dogs but I do see some really well mannered dogs playing out there and I do not want Ralphie to be one of those aggressive dogs that bite/play too rough and the other dog owners don't want their dogs to play with us anymore.