Gains or losses in stocks or other investments may not be used toward Payment Shock. To do so would be the same as saying you are counting on gains on your investments to be able to make your mortgage payment each month. Payments Shock is your way of demonstrating that you can make your full desired mortgage payment each and every month without fail, so it should be clear that depending on something as risky as stock investments to make your mortgage payment is not acceptable under any circumstances. That Payment Shock must come from a stable source of income.
Whether or not you are receiving overtime has nothing whatsoever with your Payment Shock. Again, Payment Shock is the difference between your current rent payment and your desired mortgage payment each month. Payment Shock is shown by saving at least that amount each month. How much you make has no bearing on it at all.