s = 100;
i = 0.5;
f = 0.05;
nStep = 100;
sim := NestList[If[RandomReal[] < f, # - i, # - i + 1000 #] &, s,
   nStep];

runs = Table[sim, {r, 100}];
ListPlot[runs, Joined -> True, PlotRange -> All]

ListPlot[Mean @ runs, Joined -> True, PlotRange -> All]