Step 1 is to ensure that you have SP-2 installed to Project 2010. Check File > Help and click the link "Additional Version and Copyright Information". It should clearly say SP-2 in the resulting dialog. If it does not say SP-2, get it installed and consider installing the June cumulative update as well.

Check what options you have selected in File > Options, Advanced, Calculation options. I generally leave all unchecked.

% complete is a duration measurement - so if you have split the task and hit a period of time where a resource is not working - project will reschedule the remaining work and duration around the resource's calendar.