[arbuthnot]

I'm a week and a half into Rippetoe (thanks to this forum) and I can say that the presses have been the most difficult for me as well. Mostly because it really strains my wrists even though I try to keep them pretty straight and not flexed back. Also, this last time, I felt a really deep, deep stretch in my left upper-arm through to the top of the left pectoral that I hadn't felt before.

I also think I started a bit too heavy on the press the first time, and therefore have kept it at the same weight the last two times. (115 lbs. for my work sets) This last time was still bit of a struggle to get all 5 reps in on the last set, but I did it.

I'm sure that a majority of OP's problem, as well as mine, is really no problem at all, but simply that the stabilizing muscles are just trying to catch up to the shoulders and triceps.

FWIW, my military presses have been pretty light too, 65 and 75 were my last two working set weights where I could keep good form and decent bar speed.

Also make sure that you're resting enough between sets. I often rest longer than I think I need to just so I can make sure I get my reps in and I think that helps.