I have never had this problem before, and don't really know how to handle it best. I am playing a lead opposite another actor with whom I need to do minor stages fighting an an intimate moment...and in our dress rehearsal they were drunk and stumbling. Trust - out the window - blocking - out the window.

I was so pissed I was seeing red - I'm all for following impulse and getting crazy on stage when it's called for, and if having a few shots is what someone needs to so, then I won't judge even though I find it totally unprofessional.

Unless: you're going to crash your car with me in it, if you know what I mean. I felt totally taken out of the moment, unable to complete our blocking our my direction properly because of this person...how do I go forward and do my best without feeling completely without a safety net on stage??

Should also note that this person is also the director. I have an ally in the asst dir, but the leadership isn't really in place to say something to a 'higher up' about it.

Thanks in advance for any advice...

I would just up and quit or if they are getting pissy drunk all the time.They not going to be in the right state of mind .You could turn the tables and be just as derelict.By playing pranks on stage or using lines that have nothing to do with the play.
They will see the era of there ways and if they dont .Then you need to learn to develop stronger integrity .Let this experience be a lesson ,i would just up and quit if i was you.

That is ridiculace. If you are not having fun, and you are just getting angry on stage, walk away now, don't waste any more time. From the sound of it, this show isn't gonna do your career any good... the show sounds like it will be a joke.

I must admit that I am intrigued by "JLine"s suggestion of turning the tables on them.

I threw that out there iceman because it seems the OP want's to stay with the production.It's like the old saying goes.Don't get madd get even,if these people are getting away with murder practically.Why not just flip the script,it will force them to either straighten up or .It'll give you the motivation to just quit.Which was my first answer ,wait to curtain call;then bounce.

I like it, seriously. And one could have a lot of fun throwing curve balls at during the show.

Ha ,yeah it can work to your advantage,its like holding a up a mirror to the director.That way if he/she trys to throw it back in your face.You got something to counter it with stifling that person.Also i would only advise doing if you were good with your material and improv to the point where you could deviate .Then jump back on track if need be.
I remember reading this thing on how marlon brando would sometimes would torture tallulah bankhead.She wasnt much of and actress as she was a star.So one time while she was going through a dialogue on stage performance.Brando had his back to the audience and decided to take a piss in one of the plants while she was delivering her dialogue.