I have an old Sachs coaster brake that's acting strangely. If I exert a lot of force going forward, the hub will kind of 'stick' and the pedals will try to keep rotating forward, as with a fixed gear. I can jam back on the pedals easily and make the freewheel work again, but it's annoying. I took the hub apart and put it back together and it fixed the problem for a little bit, but it came back. The torque arm and its lock nut are both secured tightly.
Did you clean out all the old grease and put in new grease? The coaster brake on a bicycle depends on the grease to operate properly. The problem you're describing sounds like old dried grease is still in the hub. Something else to think about doing is using synthetic automotive grease made for use in hubs with disk brakes. This is a very high temperature grease perfect for use in a coaster brake hub, especially if you descend long hills causing general use grease to overheat and damage the coaster brake. http://www.parktool.com/repair/readhowto.asp?id=104