If the sand is OK then it might be too much pot pressure relative to blast pressure along with incorrectly adjusted TVII valve. A big wad of media coming out when you first hit the switch usually means that the media is a build-up in the lower T fitting that continued to get pushed out through the TVII valve after the last time the main blast valve had been closed. The fact that you then don't get enough flow (after the lump is cleared out) is the TVII valve needs to be opened up. Solution is reduce pot pressure until large wad of media problem ceases then start opening TVII valve until normal desired flow appears.