Might solve overtime since you always record it, or do you? If you record the overtime you spend on internal projects it's gonna have a bad impact on your ratio of client work to internal work. What you spend 20% of your time on internal projects, your colleagues only spend 10% there - but I work 50h/week instead of 40, but no one looks on this.

Another problem is when your company doesn't have any work for you and you gonna figure out where to book the 8h of doing nothing but waiting for work that day.

Yeah working with timesheets sure is fun work.