The G-Clarons are quite easy to disassemble for cleaning. I've done a couple of 150s, a 270, and a 355, all with good results. The SKGrimes wrench is a wonderful tool for this, and paid for itself with the very first lens I cleaned. I wouldn't attempt this with a modern wide-angle lens, as the alignment is more critical.

A couple of years ago I sent a 120mm APO-Symmar to Schneider for cleaning (it had a touch of internal fungus). It was $75 plus shipping both ways.

Schneideritis is, AFAIK, a primarily cosmetic problem. I have no direct experience with it, though.