Fluctuation value detected by photodiode?

hi,i have designed an optical power meter,to measure intensity of light ...using sfh250 photodiode and the light transmitter is sfh450(plastic fiber optic connection)..actually i use pic16f84 to display the detected value in LCD..but i getting fluctuation value in lcd display..wat could be the cause? is is because the dark current element in photodiode or any other reasons?
pls help me..i'm attaching my circuitry design..so pls have a look and give me some possibilities of the fluctuation detected value..

do you have a scope to measure the frequency of the fluctuations ?
if so you may have 50 or 60hz depending of where you live.. it is the result of the light you use to see your work !