You haven't use the SQL_CALC_FOUND_ROWS.
In the manual you can find "If the preceding SELECT statement does not
include the SQL_CALC_FOUND_ROWS option, then FOUND_ROWS() may return a
different result when LIMIT is used than when it is not."
If you can, try with this option.

p.s. i use on my server mysql 5.0.18
--
Andrea