Outdoor biking Auto scroll problem

When I am out biking I like to have pulse/power or pulse/speed screen all the time on my bike mounted F5. Sometimes I can ride for 10-20 min without problems, but often the screen changes and turns black or white and jumps to first screen in activity. I have turned off auto lap, auto pause and auto scroll. Still this happens. Any idea? Is it due to dropouts from HR belt or powerpedals?