How do you report hours that vary week to week?

I work some days through a staffing agency while still certifying. I worked as an electrician's helper before this. Sometimes my paycheck is late by a week. Did anyone get an overpayment notice from a mistake?

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.

Thanks for writing this out. I went through something close to it 6 months ago, and it got easier once I kept a simple log of every call and form.