If your roof has a low pitch this may be part of your problem. If this is the case, I would consider installing an electric vent. You can leave the ridge vents open and see how much of a difference there is. Then cover the ridge vents to see if the electric vent works better when just drawing from the soffit vents. A couple of thoughts.
Insulation is designed to prevent heat flow.
If you install more attic insulation your interior may be cooler but it probably won't do much for your attic. Actually, it may make your attic hotter since there would be less heat flow to the interior of the house. This looks like tin foil and you install it with staples to the rafters inside the attic. If you are interested you can find it in a major hardware store like Home Depot or Lowes. If this is the case, I would consider installing an electric vent.