Ahhh, the good old senior slump....I miss that feeling. Actually, I think it's back again now that internship is nearly 3/4 of the way over. I just started studying for step3 and it blew my mind seeing all the things I used to know at one point or another (mostly peds and ob/gyn stuff that I haven't seen at all this year).
As for "studying" vs. relaxing before internship, you'll hear people on both side of this argument, but my advice would be to do a little of both. While reading a little bit to brush up on general medical knowledge is always a good thing, let's face it: no book can truly prepare you for your first night of call so why spend your last extended vacation ever worrying about it? Take some time to do things you may not get to do later. Backpack through Europe. Take a baseball park or roller-coaster tour of America. Work on your golf game. Learn to play guitar. Learn a new foriegn language. Whatever. You may not get another chance.
As for the fear of "not knowing enough" or the "forgetting what I've already learned" syndrome, well that's partially true but again not something to panic over. Internship is about learning as you go...if you knew it all before intern year began there would be no point in doing one. And in our field, esp. inpatient pm&r, we need all the info we learn this year to manage our future patients (hey, rehab patients can have chest pain/sob/dizziness too, in addition to the issues of spasticity, neurogenic bowel/bladder, etc. that you don't usually see in your "typical" medicine patient).
You will suffer and you will learn, but in the end hopefully you'll come out a better physician and knowing you never have to go through that year again.
I think I'll stop now. I've just realized that my soapbox has no brakes and I'm spinning out of control. Good luck to all with the match!