LFT, I was doing an ILS into DFW and the FO flying on autopilot in an MD80 when I noticed the full down deflection of glide slope and looking at DME knew things were not right so told FO to level out. We were about 1300 ft with full down glide slope about 8 miles out. I told the tower we had faulty glideslope indications because about that time it went full up and we were in the clouds. They said we are not required to protect the ILS until we are under 800 and 2 miles. A 747 was taxiing past the GS antenna and that caused the bad signal. I was a little PO'd so said should we just ask for a localizer approach from now on? Some aircraft on the ground said it is difficult to couple an approach with no working GS.