my german shepherd is aggressive to me for no reason that in can see

Question
i have a 2 1/2 year old grand champion German shepherd and for some reason he is being overly aggressive towards me. i am not the original owner i got the dog from my sister about 4 months ago. she never played with him or walked him he just sat in the back yard for the about two years she had him!! she gave the dog to me like i said four months ago he was completely untrained and disobedient.  since getting him i have taught him all the simple tricks i.e. sit,shake with both paws,lay down, stay, fetch,... and i have house broken him with no problems untill now he was just an outside dog but now he comes in allot of the time.  it seems like he listens to me 99.5 percent of the time this is all good. now my problem is every once and a while i will be sitting or laying down on the couch he will climb up with me and when i tell him to get down or push him down he will growl low and his hair will stand up and will snap at me some times. this is not an every day thing he will get up and chill with me and i will get him off the couch with no proplems. it seems like it only happens when i have ben drinking and his head is higher than mine.  this may be a dominice thing??? but i dont know why he listens to me and does ever thing i say 99.5 percent of the time but growls and snaps at my face on occasian i have room mates they dont correct him i do that all and they have never experenced this problem. he weighs about 125 pounds he is huge so this would be a big problem if he ever decided to do more than a snap i love my dog and would like to correct this problem!!!!! any tips or insite to why this is happening and what i can do to correct and prevent this from happening any further would be awesome

Answer
He's a poorly behaved Grand Champion? What area is he a champion in? First of all don't let him on the couch period. Alpha dogs only have the highest sleeping place. He shouldn't be sharing yours at all. Secondly, get him into an obedience class asap. Thirdly, walk him, walk him, walk him. He needs this to keep him exercised, and to establish dominance. GSD's are working dogs and need a job,have him carry a pack for you when you walk. Also in your browsers search window type in Alpha Boot Camp. Memorize it :). Then implement it . DO get the obedience classes underway immediately. If he isn't neutered, and you don't plan to breed him, doing so immediately might also help. Have him work for his food. Teach him to sit before eating, before petting etc. He should be earning any rewards he gets.
Let me know how he does
Cindi