After capturing a SBP ILS 11 false glideslope; a PA-31 pilot executed a missed approach; later discovering the false glideslope is a known problem.

Narrative

The weather conditions were at ILS approach minimums at San Luis Obispo; CA and I was cleared for the ILS Runway 11 approach. I was cleared for the approach; told to maintain 2500 until established. Once cleared for the approach; I intercepted the localizer course and glideslope normally. Nothing seemed out of place. As I passed over the outer marker (DOBRA intersection LOM) I checked my altitude and noticed it was significantly off. I should have been at 2182'. Instead I was at only 1900' and perfectly on 'glideslope'. I immediately executed a go-around and queried tower. I confirmed the altimeter setting and it was correctly set. It seems I was on a 'false glideslope' which was slightly lower than the actual glideslope. I came back around for another approach and this time the altitude checked at the FAF and the approach went normally (although I was issued a 'low-altitude alert' by tower despite being established perfectly on the correct glideslope this time). After landing; I did some research and a local flight instructor said that this 'false glideslope' is common at that airport and that they do know about it and can't fix it. I called the ATC tower to report and the Controller also seemed aware of the problem; and expected that was why I went missed. That morning; I had checked the NOTAM which warned 'check altitude at DOBRA 2182' and I think this made it a very easy decision to go-around when I saw a different reading nearly 300 FT off. HOWEVER; I think the NOTAM could have at least been worded more strongly to convey the problem; or this problem should be included on the IAP chart itself if the problem cannot be corrected. Apparently; other air carriers (including a passenger air carrier have experienced the same problem and executed a go-around). The danger of this problem is that it is very subtle and it takes an alert and competent instrument pilot to even realize there is a problem... This could be very dangerous if someone neglected to cross-check altitude at the Outer Marker and found themselves unknowingly on a low false glideslope!