First off all you shouldn't be shifting to the extremes. If your in the largest ring in the front I usually go 4 or higher in the back with a 7 speed cog. Thats probably normal, when I'm in 2 in the front and 1 in the back my chain will jump down a cog on the back when I pedal backwards. Likewise if your in the smallest ring up front and the smallest out back you might have problems. Hope that helps