Yes, the problem occurs most frequently around the layer transition as you say. Its a known problem with a very large number of SD-612's. Ive tried almost every firmware I can get my hands on with my drive in vane. Some firmware versions (ss11u in my case) seem to be particularly bad, preventing me from even playing some DVD's at all, while other DVD's work fine with it. SS07 works better than most, tho many disks choke at random points on the disk, and very often at the layer change. Sumsung sadly seem to be very quiet about all this. tbh it seems like they are just very dodgy bad drives.
Id be interested however, to hear how other ppl have got on... David