No worries, a sample would help but a lot of the time we cant do that
The items you mention shouldn't make any difference- project start date wont change calendars, no matter what your start date is, nor should lag as this wil use working days, and not create a new data set.
Only thing I can think of is that somewhere the standard calendar has become a 7 day calendar (or some other user created one). you said you used an older file to create a new one, its possible something happened there and things went wonky.
If you haven't already, double check you calendars. Insert a new column in the programme temporarily and use "task calendar" to see what each task is usung. I suspect that all tasks are using the same calendar. If its not standard, you can change them over and hopefully that should cure it. If not, check the calendar in use to see what days is uses- if there is anything in sat or sun, then delete it, and that should get you back to a 5 day week.
If that doesnt work, I'll see what else we can come up with