Your description generally indicates a bad fiber optic cable. The required cable should meet the following specifications:
Wavelength: 820 nanometers typical.
Connectors: SMA-905 series.
Required Cable: Duplex 100ìm, 140ìm clad fiber optic.
200MHz-km bandwidth; 6 db/km max. attenuation.
Max. Cable Length: 1000 meters.
Data Rates: 3400 bytes per second typical.
I suggest you contact your local distributor or if in the USA, contact Measurement Computing for technical assistance.