I had the same symptoms earlier this year but mine turned out to be simply low on fluid. It seems that when you go up and down hills the big 'mentos' looking thin in the reservoir moves and when it is low it doesn't touch the sensor on the under side of the lid.
Maybe try wiggle the sensor under lid to see it i'snt staying stuck out, worth a shot.
My other guess is have you had anything done to the front wheels lately? ie: bushes, bearings, shocks, as i read a lot of warnings about being careful with the abs sensors as they can easily break? if so it could be one of these.