Many pear varieties do this if left to ripen fully on the tree - you need to pick them earlier, before they're obviously ripe. They ripen from the inside out, so the inner core is ripened before the exterior of the fruit, so if you wait till they're ripened on the outside, the inside is already past the point of no return and starting to rot. The right time for picking varies year on year, depending on the weather - this year they would likely have been ready earlier than last year. Try picking them earlier and keeping them indoors somewhere while they finish ripening.