problem in generating positive stable random numbers

From where did you get rstable()? It isn't part of R itself, and

poking around shows at least three different sources for it
(CircStats, fBasics, stable (Lambert/Lindsey, not on CRAN)). I seem to
recall that the third one also has a distribution function which is
almost, but not quite monotone.

The usual advice of taking package problems to the package maintainer
applies.