Siobhan, I’m not sure what you mean by broken tasks but I’ll respond based on my assumptions. What version are you using? There have been several bugs in 2007 and some in earlier versions of 2010 that will inaccurately sync timesheet actuals to the project schedule. I have not heard of this as a problem in 2013 and certainly not to the extent that you are inferring. If the issue is limited, you could temporarily uncheck the “Only allow task updates via Tasks and Timesheets” setting and manually fix the issues directly in Microsoft Project. I recommend doing this off-hours to prevent other problems, and make sure you recheck the checkbox when you are done. If it is happening consistently and growing, I would suggest submitting a ticket with Microsoft.