There's one hill on my regular ride (trail - not commute) that I have the same problem with. The frustrating part is that I've cleaned it a couple of times, but I can't for the life of me remember which magic gear combo. did it for me. I've tried several times since then in different gears and I either can't smash hard enough or the rear breaks loose, but everything's been covered in slick dead leaves lately, so maybe that's the critical difference.