I have experienced the same problem with the glideslope on 26R with VS going up and down beteween 400-1000ft/min with no apparent reason. One of the times the ride became very uncomfortable and i just took the autopilot out and continued visually. When we landed we told the controller but the answer we got was only that he would report it. No news on it. I guess next time i will file an ASR and let the company deal with it.