David Nagel has resigned as PalmSource chief executive, president and director, a move that follows a series of executive changes in the past several months at the struggling maker of the Palm handheld operating system. Nagel, who resigned as of Sunday, has been replaced by Patrick McVeigh, PalmSource senior vice president of worldwide licensing and sales. McVeigh will serve as interim CEO as the company initiates a search for a permanent replacement. Nagel will remain as a company adviser through mid-July to help with the transition.