Arrgh... I can empathise with strangely spaced studs. My house was built in 1860, apparently before they had invented standards.

When I recently tried to put a small shelf in my bathroom, I ended up cutting away a horizontal inch-wide strip of plaster and lathe until I located the 2 studs, and then fashioning a wooden cross brace to fix to the studs, sitting nearly flush with the wall. I then spackled over the top, and fixed the shelf into the brace. It was the only way I could find to put the shelf where I actually needed it.