Update downloads keep stopping.......

For some reason, whenever I try to get updates (either via Software Update or by manually downloading the needed updates from the Apple site), they always stop (sometimes @ 5 MB, sometimes @ 9 MB, or whatever MB). Has anyone else experienced this?? I have no problems updating the PC that is in the same room as my Mac. In fact, it is downloading & installing updates right now for iTunes & QuickTime without any issues. This has me puzzled. My internet service provider verified the line & found no packet losses. Any input will be greatly appreciated. Thanks in advance.