Mine does the same thing when I stop at stop signs or lights but goes back up to between the 3rd and 4th line once I start moving. I am assuming that this is normal. Maybe others have a different opinion.
I had the same concern when I first purchased a few months back. It would go from around 1/4 pressure at a stop to just over 1/2 way when I accelerated. Made me very nervous but posted this same concern and had an overwhelming replies from folks stating that it was completely normal.