I have been having this same issue for almost a week. My problem started when I purchased a pair of jean from Robbish and put them on.
Sometime I can log in, do the test male, and am fine when I switch back to my usual self, but most times I go back to invisible with the first change. I have a local OSGrid server than I run once in a while to build with, and do NOT have the same problem there.
It makes no difference if I use primary monitor or secondary monitor, remove all clothing, or even if I log in with metabolt (a non-graphical viewer), phoenix, firestorm or SL. I have tried about every thing I have found in other forum posts, and a few things not listed, such as deleting my normally worn inventory item, and re-unboxing them. Nothing helps this.
HELP!! I am almost ready to leave SL because of this, but I hate to give up all my friends and the money I have invested!