If the pads or disks are still quite new it might be worth taking the wheels off and having a good poke round to make sure everythings still present and where it should be. Also, could something have got stuck between the disk and the caliper like a piece of grit or salt that only catches when in a certain position? I once had a piece of grit stuck on my caliper with tar after a road had been re surfaced. As for the ABS, the best way I've found to describe it is it's like braking on gravel or marbles and the pedal feels as if it's trying to push back a bit, you wonder what the hell is going on the first time it gets you.