A PA-28 pilot descended on the false glide slope on the MFR Runway 14 ILS; before the Final Approach Fix. TRACON issued a low altitude alert with a climb and vectors for another approach.

Narrative

I was vectored for an ILS to Runway 14 at Medford; Oregon. After being cleared for the approach and intercepting the localizer; I observed the glide slope indicator showing glide slope interception. I began decent and tracked the glide slope and localizer about 2 miles prior to reaching the actual glide slope intercept as depicted on the approach plate. I decended 500 FT to an altitude of 4;500 FT at about AMASE (12.7 DME). Cascade Approach called to inform me that I was too low and to cross AMASE at 5;000 FT. I realized that the glide slope indication was false and began a climb. I then was instructed to climb to 6;300 FT and given vectors for another approach. On the second approach I used the localizer minimums. The false glide slope indication did not show an 'OFF' flag. I believe that the ILS/VOR receiver installed in the plane was very old and was not receiving signals adequately for navigation. The next day; in VFR weather I could not duplicate the problem. The aircraft owner; also a pilot; was in the right seat and also observed the false reading.